Guaranteed data compression

ABSTRACT

Lossy methods and hardware for compressing data and the corresponding decompression methods and hardware are described. The lossy compression method comprises dividing a block of pixels into a number of sub-blocks and then analysing, for each sub-block, and selecting one of a candidate set of lossy compression modes. The analysis may, for example, be based on the alpha values for the pixels in the sub-block. In various examples, the candidate set of lossy compression modes comprises at least one mode that uses a fixed alpha channel value for all pixels in the sub-block and one or more modes that encode a variable alpha channel value.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

This application is a continuation under 35 U.S.C. 120 of copendingapplication Ser. No. 16/457,958 filed Jun. 29, 2019, which claimsforeign priority under 35 U.S.C. 119 from United Kingdom Application No.1810794.6 filed Jun. 29, 2018, the disclosures of which are incorporatedherein in their entireties.

BACKGROUND

Data compression, either lossless or lossy, is desirable in manyapplications in which data is to be stored in, and/or read from, amemory. By compressing data before storage of the data in a memory, theamount of data transferred to the memory may be reduced. An example ofdata for which data compression is particularly useful is image data,such as depth data to be stored in a depth buffer, pixel data to bestored in a frame buffer and texture data to be stored in a texturebuffer. These buffers may be any suitable type of memory, such as cachememory, separate memory subsystems, memory areas in a shared memorysystem or some combination thereof.

A Graphics Processing Unit (GPU) may be used to process image data inorder to determine pixel values of an image to be stored in a framebuffer for output to a display. GPUs usually have highly parallelisedstructures for processing large blocks of data in parallel. There issignificant commercial pressure to make GPUs (especially those intendedto be implemented on mobile devices) operate at lower power levels.Competing against this is the desire to use higher quality renderingalgorithms on faster GPUs, which thereby puts pressure on a relativelylimited resource: memory bandwidth. However, increasing the bandwidth ofthe memory subsystem might not be an attractive solution because movingdata to and from, and even within, the GPU consumes a significantportion of the power budget of the GPU. The same issues may be relevantfor other processing units, such as central processing units (CPUs), aswell as GPUs.

The embodiments described below are provided by way of example only andare not limiting of implementations which solve any or all of thedisadvantages of known methods of data compression.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

Lossy methods and hardware for compressing data and the correspondingdecompression methods and hardware are described. The lossy compressionmethod comprises dividing a block of pixels into a number of sub-blocksand then analysing, for each sub-block, and selecting one of a candidateset of lossy compression modes. The analysis may, for example, be basedon the alpha values for the pixels in the sub-block. In variousexamples, the candidate set of lossy compression modes comprises atleast one mode that uses a fixed alpha channel value for all pixels inthe sub-block and one or more modes that encode a variable alpha channelvalue.

A first aspect provides a lossy method of compressing data comprising:dividing a block of pixels into a plurality of sub-blocks; and for eachsub-block: analysing an alpha channel value for each pixel in thesub-block to select a lossy compression mode from a set of candidatelossy compression modes, wherein the set of candidate lossy compressionmodes comprises two or more lossy compression modes including at leastone compression mode which uses a fixed alpha channel value for allpixels in the sub-block; and compressing the sub-block using theselected lossy compression mode.

A second aspect provides a method of data decompression comprising:unpacking data from a compressed data block to generate compressed datafor a plurality of sub-blocks; for each sub-block: identifying acompression mode from a block mode field within the compressed datablock, wherein the compression mode is one of a set of candidate lossycompression modes, the set of candidate lossy compression modescomprising two or more lossy compression modes including at least onecompression mode which uses a fixed alpha channel value for all pixelsin the sub-block; and decompressing the sub-block using a decompressionmode corresponding to the selected compression mode.

A third aspect provides a data compression unit comprising: hardwarelogic arranged to dividing a block of pixels into a plurality ofsub-blocks; hardware logic arranged, for each sub-block, to analyse analpha channel value for each pixel in the sub-block to select a lossycompression mode from a set of candidate lossy compression modes,wherein the set of candidate lossy compression modes comprises two ormore lossy compression modes including at least one compression modewhich uses a fixed alpha channel value for all pixels in the sub-block;and a data compression element arranged to compress the sub-block usingthe selected lossy compression mode.

A fourth aspect provides a data decompression unit comprising: hardwarelogic arranged unpack data from a compressed data block to generatecompressed data for a plurality of sub-blocks; hardware logic arranged,for each sub-block, to identify a compression mode from a block modefield within the compressed data block, wherein the compression mode isone of a set of candidate lossy compression modes, the set of candidatelossy compression modes comprising two or more lossy compression modesincluding at least one compression mode which uses a fixed alpha channelvalue for all pixels in the sub-block; and a data decompression elementarranged to decompress the sub-block using a decompression modecorresponding to the selected lossy compression mode.

The data compression and/or decompression unit as described herein maybe embodied in hardware on an integrated circuit. There may be provideda method of manufacturing, at an integrated circuit manufacturingsystem, a data compression and/or decompression unit as describedherein. There may be provided an integrated circuit definition datasetthat, when processed in an integrated circuit manufacturing system,configures the system to manufacture a data compression and/ordecompression unit as described herein. There may be provided anon-transitory computer readable storage medium having stored thereon acomputer readable description of an integrated circuit that, whenprocessed, causes a layout processing system to generate a circuitlayout description used in an integrated circuit manufacturing system tomanufacture a data compression and/or decompression unit as describedherein.

There may be provided an integrated circuit manufacturing systemcomprising: a non-transitory computer readable storage medium havingstored thereon a computer readable integrated circuit description thatdescribes the data compression and/or decompression unit as describedherein; a layout processing system configured to process the integratedcircuit description so as to generate a circuit layout description of anintegrated circuit embodying the data compression and/or decompressionunit as described herein; and an integrated circuit generation systemconfigured to manufacture the data compression and/or decompression unitas described herein according to the circuit layout description.

There may be provided computer program code for performing any of themethods described herein. There may be provided non-transitory computerreadable storage medium having stored thereon computer readableinstructions that, when executed at a computer system, cause thecomputer system to perform any of the methods described herein.

The above features may be combined as appropriate, as would be apparentto a skilled person, and may be combined with any of the aspects of theexamples described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described in detail with reference to theaccompanying drawings in which:

FIG. 1 shows a graphics rendering system;

FIGS. 2A, 2B and 2C show three different data compression architectures;

FIG. 3A is a flow diagram of an example lossy data compression method;

FIG. 3B is a flow diagram of a data decompression method that may beused to decompress data that was compressed using the method of FIG. 3A;

FIGS. 4A and 4B are schematic diagram showing different blocks of dataand their subdivision into sub-blocks;

FIG. 4C is a schematic diagram showing an example compressed data block;

FIGS. 5A and 5B show two different example implementations of theanalysis stage of the method of FIG. 3A;

FIG. 6A is a flow diagram of a first example method of compressing asub-block using the constant alpha mode of FIG. 3A;

FIGS. 6B, 6C and 6D are schematic diagrams showing two different ways ofpacking the compressed values into a data block;

FIG. 7A is a flow diagram of a second example method of compressing asub-block using the constant alpha mode of FIG. 3A;

FIG. 7B is a schematic diagram showing an example of how the pixels in asub-block may be divided into the two subsets in the method of FIG. 7A;

FIG. 8A is a flow diagram of a first example method of compressing asub-block using the variable alpha mode of FIG. 3A;

FIG. 8B is a schematic diagram illustrating a part of the method of FIG.8A;

FIG. 9 is a flow diagram of a second example method of compressing asub-block using the variable alpha mode of FIG. 3A;

FIG. 10A is a schematic diagram showing encoding patterns that may beused in the method of FIG. 9;

FIGS. 10B and 100 are schematic diagrams showing two different ways inwhich compressed data for a mini-block is packed into a data field;

FIG. 10D is a schematic diagram showing two further encoding patternsthat may be used in the method of FIG. 9;

FIG. 11 is a flow diagram of a further example method of compressing asub-block using a constant alpha mode;

FIG. 12A is a flow diagram of a method of converting an n-bit number toan m-bit number, where n>m;

FIG. 12B is a schematic diagram of a hardware implementation of themethod of FIG. 12A;

FIG. 13A is a flow diagram of a method of converting an n-bit number toan m-bit number, where n<m;

FIG. 13B is a schematic diagram of a hardware implementation of themethod of FIG. 13A;

FIGS. 13C and 13D are schematic diagrams illustrating two examples ofthe method of FIG. 13A;

FIG. 14 is a flow diagram of a first example method of converting 10-bitdata to 8-bit data;

FIGS. 15A, 15B, 15C and 15D are schematic diagrams showing fourdifferent ways in which data may be packed into data fields dependentupon whether the methods of FIG. 9 or 11 are used;

FIG. 16 is a flow diagram of a first example method of converting 10-bitdata to 8-bit data;

FIG. 17 is a schematic diagram that illustrates the method of FIG. 16;

FIG. 18A is a flow diagram of a data compression method which combinesthe pre-processing method of FIG. 16 with a lossless data compressionmethod;

FIG. 18B is a flow diagram of a data decompression method that maybeused where data has been compressed using the method of FIG. 18A;

FIG. 19A is a schematic diagram of a further example data compressionunit;

FIG. 19B is a flow diagram of a method of operation of the bit predictorelement in the data compression unit shown in FIG. 19A;

FIG. 19C is a schematic diagram showing an example way in which data maybe packed into a compressed data block where the method of FIG. 19A isused;

FIG. 20A shows a schematic diagram of another data compression unit;

FIG. 20B is a flow diagram of a method of lossless data compression;

FIG. 21A is a schematic diagram of a block of data that has beenspatially decorrelated and remapped using the hardware of FIG. 20A;

FIG. 21B is a schematic diagram showing an encoded data output from themethod of FIG. 22;

FIG. 22 is a flow diagram showing a method of entropy encoding;

FIG. 23 shows a computer system in which a data compression and/ordecompression unit is implemented; and

FIG. 24 shows an integrated circuit manufacturing system for generatingan integrated circuit embodying a data compression and/or decompressionunit as described herein.

The accompanying drawings illustrate various examples. The skilledperson will appreciate that the illustrated element boundaries (e.g.,boxes, groups of boxes, or other shapes) in the drawings represent oneexample of the boundaries. It may be that in some examples, one elementmay be designed as multiple elements or that multiple elements may bedesigned as one element. Common reference numerals are used throughoutthe figures, where appropriate, to indicate similar features.

DETAILED DESCRIPTION

The following description is presented by way of example to enable aperson skilled in the art to make and use the invention. The presentinvention is not limited to the embodiments described herein and variousmodifications to the disclosed embodiments will be apparent to thoseskilled in the art.

Embodiments will now be described by way of example only.

As described above, memory bandwidth is a relatively limited resourcewithin a processing unit (e.g. a CPU or GPU), similarly, memory space isa limited resource because increasing it has implications in terms ofboth physical size of a device and power consumption. Through the use ofdata compression before storage of data in a memory, both the memorybandwidth and the space in memory are reduced.

Many data compression schemes exist, some of which are lossless andothers that are lossy. Lossless compression techniques may be preferredin some situations because the original data can be perfectlyreconstructed from the compressed data. In contrast, where lossycompression techniques are used, data cannot be perfectly reconstructedfrom the compressed data and instead the decompressed data is only anapproximation of the original data. The accuracy of the decompressed(and hence reconstructed) data will depend upon the significance of thedata that is discarded during the compression process. Additionally,repeatedly compressing and decompressing data using lossy compressiontechniques results in a progressive reduction in quality, unlike wherelossless compression techniques are used. Lossless compressiontechniques are often used for audio and image data and examples ofgeneral purpose lossless compression techniques include run-lengthencoding (RLE) and Huffman coding.

The amount of compression that can be achieved using losslesscompression techniques (e.g. as described in UK patent number 2530312)depends on the nature of the data that is being compressed, with somedata being more easily compressed than other data. The amount ofcompression that is achieved by a compression technique (whetherlossless or lossy) may be expressed in terms of a percentage that isreferred to herein as the compression ratio and is given by:

${{Compression}\mspace{14mu}{ratio}} = {\frac{{Compressed}\mspace{14mu}{size}}{{Uncompressed}\mspace{14mu}{size}} \times 100}$It will be appreciated that there are other ways to define thecompression ratio; however, the above convention is used throughout.This means that a compression ratio of 100% indicates that nocompression has been achieved, a compression ratio of 50% indicates thatthe data has been compressed to half of its original, uncompressed sizeand a compression ratio of 25% indicates that the data has beencompressed to a quarter of its original, uncompressed size. Lossycompression techniques can typically compress data to a greater extent(i.e. achieve smaller compression ratios) than lossless compressiontechniques. Therefore, in some examples, e.g. where the extent ofachievable compression is considered more important than the quality ofthe decompressed (i.e. reconstructed) data, lossy compression techniquesmay be preferred over lossless compression techniques. The choicebetween a lossless and a lossy compression technique is animplementation choice.

The variability in the amount of compression that can be achieved (whichis dependent upon characteristics of the actual data that is beingcompressed) has an impact on both memory bandwidth and memory space andmay mean that the full benefit of the compression achieved is notrealised in relation to one or both of these two aspects, as describedbelow.

In many use cases, random access of the original data is required.Typically for image data, to achieve this, the image data is dividedinto independent, non-overlapping, rectangular blocks prior tocompression. If the size of each compressed block varies because of thenature of the data in the block (e.g. a block which is all the samecolour may be compressed much more than a block which contains a lot ofdetail) such that in some cases a block may not be compressed at all,then in order to maintain the ability to randomly access the compresseddata blocks, the memory space may be allocated as if the data was notcompressed at all. Alternatively, it is necessary to maintain an index,with an entry per block that identifies where the compressed data forthat block resides in memory. This requires memory space to store theindex (which is potentially relatively large) and the memory accesses(to perform the look-up in the index) adds latency to the system. Forexample, in systems where it is important to be able to randomly accesseach compressed block of data and where an index is not used, even if anaverage compression ratio (across all data blocks) of 50% is achieved,memory space still has to be allocated assuming a 100% compressionratio, because for some blocks it may not be possible to achieve anycompression using lossless compression techniques.

Furthermore, as the transfer of data to memory occurs in fixed sizebursts (e.g. in bursts of 64 bytes), for any given block there are onlya discrete set of effective compression ratios for the data transfer tomemory. For example, if a block of data comprises 256 bytes and thetransfer of data occurs in 64 byte bursts, the effective compressionratios for the data transfer are 25% (if the block is compressed from256 bytes to no more than 64 bytes and hence requires only a singleburst), 50% (if the block is compressed into 65-128 bytes and hencerequires two bursts), 75% (if the block is compressed into 129-192 bytesand hence requires three bursts) and 100% (if the block is notcompressed at all or is compressed into 193 or more bytes and hencerequires four bursts). This means that if a block of data comprising 256bytes is compressed into anywhere in the range of 129-192 bytes, thenthree bursts are required for the compressed block, compared to four forthe uncompressed block, making the effective compression ratio for thememory transfer 75% whilst the actual data compression achieved could bemuch lower (e.g. as low as 50.4% if compressed into 129 bytes).Similarly, if the compression can only compress the block into 193bytes, the memory transfer sees no benefit from the use of datacompression, as four bursts are still required to transfer thecompressed data block to memory. In other examples, blocks of data maycomprise a different number of bytes, and bursts to memory may comprisea different number of bytes.

Described herein are various methods of performing data compression.Some of the methods described herein provide a guarantee that acompression threshold, which may be defined in terms of a compressionratio (e.g. 50%), compressed block size (e.g. 128 bytes) or in any otherway, is met. An effect of this guarantee is that a reduced amount ofmemory space can be allocated whilst still enabling random access toblocks of compressed data and there is also a guaranteed reduction inthe memory bandwidth that is used to transfer the compressed data to andfrom memory. In other examples the compression ratio may be targeted(i.e. the method may be configured to achieve the ratio in the majorityof cases) but there is no guarantee that it will be met.

Also described herein are methods for converting 10-bit (e.g.10:10:10:2) data to 8-bit (e.g. 8:8:8:3) data and methods for mappingfrom an n-bit number to an m-bit number. As described below, the methodsfor converting 10-bit (e.g. 10:10:10:2) data to 8-bit (e.g. 8:8:8:3 or8888) data may be used as a pre-processing (or pre-encoding) step forthe methods of performing data compression described herein or may beused independently (e.g. with another data compression method or withonly a lossless compression method, such as that described below withreference to FIGS. 20A-B, 21A-B and 22). By first converting the 10-bit(e.g. 10:10:10:2) data using one of the methods described herein, the10-bit can then subsequently be compressed by methods that are arrangedto operate on 8888 format data. The conversion method may be lossy withrespect to three of the channels (e.g. the RGB data) and lossless forthe fourth channel (e.g. the alpha data); however as this format istypically used for high dynamic range (HDR) data and the majority ofpixels (e.g. 75%) will still be of low dynamic range (LDR), theconversion can be performed with only a small loss of accuracy. Themethod for mapping from an n-bit number to an m-bit number describedherein may be used within the methods of performing data compression asdescribed below or may be used independently. By using this mappingmethod, data of other formats can be subsequently compressed by methodsthat are arranged to operate on 8888 format data and/or it can be usedto reduce the internal buffering (e.g. registers, etc.) by, for example,6 bits per pixel (i.e. 19%) and this may, for example, be used in theinitial reserve compression sub-unit 204A described below and shown inFIG. 2C.

FIG. 1 shows a graphics rendering system 100 that may be implemented inan electronic device, such as a mobile device. The graphics renderingsystem 100 comprises a host CPU 102, a GPU 104 and a memory 106 (e.g. agraphics memory). The CPU 102 is arranged to communicate with the GPU104. Data, which may be compressed data, can be transferred, in eitherdirection, between the GPU 104 and the memory 106.

The GPU 104 comprises a rendering unit 110, a compression/decompressionunit 112, a memory interface 114 and a display interface 116. The system100 is arranged such that data can pass, in either direction, between:(i) the CPU 102 and the rendering unit 110; (ii) the CPU 102 and thememory interface 114; (iii) the rendering unit 110 and the memoryinterface 114; (iv) the memory interface 114 and the memory 106; (v) therendering unit 110 and the compression/decompression unit 112; (vi) thecompression/decompression unit 112 and the memory interface 114; and(vii) the memory interface 114 and the display interface. The system 100is further arranged such that data can pass from thecompression/decompression unit 112 to the display interface 116. Images,which are rendered by the GPU 104, may be sent from the displayinterface 116 to a display for display thereon.

In operation, the GPU 104 processes image data. For example, therendering unit 110 may perform scan conversion of graphics primitives,such as triangles and lines, using known techniques such asdepth-testing (e.g. for hidden surface removal) and texturing and/orshading. The rendering unit 110 may contain cache units to reduce memorytraffic. Some data is read or written by the rendering unit 110, to thememory 106 via the memory interface unit 114 (which may include a cache)but for other data, such as data to be stored in a frame buffer, thedata preferably goes from the rendering unit 110 to the memory interface114 via the compression/decompression unit 112. Thecompression/decompression unit 112 reduces the amount of data that is tobe transferred across the external memory bus to the memory 106 bycompressing the data, as described in more detail below.

The display interface 116 sends completed image data to the display. Anuncompressed image may be accessed directly from the memory interfaceunit 114. Compressed data may be accessed via thecompression/decompression unit 112 and sent as uncompressed data to thedisplay 108. In alternative examples the compressed data could be sentdirectly to the display 108 and the display 108 could include logic fordecompressing the compressed data in an equivalent manner to thedecompression of the compression/decompression unit 112. Although shownas a single entity, the compression/decompression unit 112 may containmultiple parallel compression and/or decompression units for enhancedperformance reasons.

In various examples, the compression/decompression unit 112 mayimplement a compression method (or scheme) that guarantees that acompression threshold (which may be pre-defined and hence fixed or maybe an input variable) is met. As detailed above, the compressionthreshold may, for example, be defined in terms of a compression ratio(e.g. 50% or 25%), compressed block size (e.g. 128 bytes) or in anyother way. In order to provide this guarantee in relation to the amountof compression that is provided, and given that the exact nature of thedata is not known in advance, a combination of lossless and lossycompression methods are used and three example architectures are shownin FIGS. 2A-C. In most if not all cases, a lossless compressiontechnique (such as that described in UK patent number 2530312 or asdescribed below with reference to FIGS. 20A-B, 21A-B and 22) is used tocompress a block of data and then a test is performed to determinewhether the compression threshold is met. In the event that thecompression threshold is not met, a lossy compression technique (such asvector quantisation (VQ) techniques) or the method described below withreference to FIGS. 3A and 4-11 that provides the guaranteed compressionaccording to the compression threshold) is instead applied to the datablock to achieve the compression threshold.

In the method shown in FIG. 2A, the uncompressed source data, (e.g. ablock of 256 bytes) is input to both a primary compression unit 202(which may also be referred to as a lossless compression unit) and areserve compression unit 204 (which may also be referred to as a lossyor fallback compression unit). The input data block is thereforeindependently and in parallel compressed using two different methods (apotentially lossless method in the primary compression unit 202 and alossy method in reserve compression unit 204). An example method oflossless compression that may be implemented by the primary compressionunit 202 is described below with reference to FIGS. 20A-B, 21A-B and 22.The reserve compression unit 204 compresses the input data block in sucha way so as to guarantee that the compression threshold is satisfied.The two versions of the compressed data block are then input to a testand selection unit 206. This test and selection unit 206 determineswhether the compressed data block generated by the primary compressionunit 202 satisfies the compression threshold (e.g. if it is no largerthan 128 bytes for a 256 byte input block and a 50% compressionthreshold). If the compressed data block generated by the primarycompression unit 202 satisfies the compression threshold, then it isoutput, otherwise the compressed data block generated by the reservecompression unit 204 is output. In all cases the compressed data blockthat is output satisfies the compression threshold and by only usinglossy compression (in the reserve compression unit 204) for those blocksthat cannot be suitably compressed using lossless techniques (in theprimary compression unit 202), the overall quality of the compresseddata is improved (i.e. the amount of data that is lost due to thecompression process is kept low whilst still satisfying the compressionthreshold).

In the method shown in FIG. 2B, the uncompressed source data, (e.g. ablock of 256 bytes) is initially input to only the primary compressionunit 202 and the input of the source data to the reserve compressionunit 204 is delayed (e.g. in delay unit 208). The amount of delay may bearranged to be similar to the time taken to compress the source datablock using the lossless compression technique (in the primarycompression unit 202) or a little longer than this to also include thetime taken to assess the size of the compressed data block output by theprimary compression unit 202 (in the test and decision unit 210). Thecompressed data block output by the primary compression unit 202 isinput to the test and decision unit 210 and if it satisfies thecompression threshold it is output and no lossy compression isperformed. If, however, the compressed data block output by the primarycompression unit 202 does not satisfy the compression threshold (i.e. itis still too large), then the test and decision unit 210 discards thiscompressed block and triggers the lossy compression of the block by thereserve compression unit 204. The compressed data block output by thereserve compression unit 204 is then output.

In the method shown in FIG. 2C, the reserve compression unit 204 isdivided into two sub-units: an initial reserve compression sub-unit 204Aand a final reserve compression sub-unit 204B, with each sub-unitperforming a part of the lossy compression method. For example, theinitial reserve compression sub-unit 204A may compress each byte from 8bits to 5 bits (e.g. using truncation or the method described below withreference to FIGS. 12A-B) and any further compression that is requiredto satisfy the compression threshold may be performed by the finalreserve compression sub-unit 204B. In other examples, the reservecompression sub-unit 204B may perform a pre-processing step, (e.g. asdescribed below with reference to FIG. 14). In yet further examples, thelossy compression method may be split in different ways between the tworeserve compression sub-units 204A, 204B.

In the method shown in FIG. 2C, the uncompressed source data, (e.g. ablock of 256 bytes) is input to both the primary compression unit 202and the initial reserve compression sub-unit 204A. The input data blockis therefore independently and in parallel compressed using twodifferent methods (a lossless method in the primary compression unit 202and the first part of a lossy method in sub-unit 204A). The compresseddata block output by the primary compression unit 202 is input to thetest and decision unit 210 and if it satisfies the compression thresholdit is output, the partially compressed data block output by the initialreserve compression sub-unit 204A is discarded and no further lossycompression is performed for that data block. If, however, thecompressed data block output by the primary compression unit 202 doesnot satisfy the compression threshold (i.e. it is still too large), thenthe test and decision unit 210 discards this compressed block andtriggers the completion of the lossy compression of the block output bythe initial reserve compression sub-unit 204A by the final reservecompression sub-unit 204B. The compressed data block output by the finalreserve compression sub-unit 204B is output.

In certain situations, it may be possible to compress a data block bymore than the compression threshold. In such instances, the primarycompression unit 202 may output a compressed data block that alwaysexactly satisfies the compression threshold or alternatively, the sizeof the output compressed data block may, in such situations, be smallerthan that required to satisfy the compression threshold. Similarly, thelossy compression technique that is used in FIGS. 2A, 2B and 2C (andimplemented in the reserve compression unit 204 or sub-units 204A, 204B)may output a compressed data block which always exactly satisfies thecompression threshold or alternatively, the size of the compressed datablock may vary whilst still always satisfying the compression threshold.In the case where a compressed data block is smaller than is required toexactly satisfy the compression threshold, there may still be memorybandwidth and memory space inefficiencies caused by fixed burst sizesand pre-allocation requirements respectively; however, as thecompression threshold is satisfied, there is always an improvement seenin relation to both memory bandwidth and memory space. In variousexamples, headers may be used to reduce the used memory bandwidth forsome blocks even further (e.g. by including in the header informationabout how much data to read from memory or write to memory).

Depending upon the particular implementation, any of the architecturesof FIGS. 2A-C may be used. The arrangement shown in FIG. 2A provides afixed throughput and fixed latency (which means that no buffering ofdata is needed and/or no bubbles are caused later in the system) but thepower consumption may be increased (e.g. compared to just having asingle compression unit performing either lossless or lossycompression). The arrangement shown in FIG. 2B may have a lower powerconsumption (on average) than the arrangement shown in FIG. 2A becausethe reserve compression unit 204 in FIG. 2B can be switched off when itis not needed; however the latency may vary and as a result buffers maybe included in the system. Alternatively, an additional delay element208 (shown with a dotted outline in FIG. 2B) may be added between thetest and decision unit 210 and the output to delay the compressed datablock output by the primary compression unit 202 (e.g. the amount ofdelay may be arranged to be comparable to the time taken to compress thesource data block using the lossy compression technique in the reservecompression unit 204). The inclusion of this additional delay element208 into the arrangement of FIG. 2B has the effect of making the latencyof the arrangement fixed rather than variable. The arrangement shown inFIG. 2C may also have a lower power consumption (on average) than thearrangement shown in FIG. 2A because the final reserve compressionsub-unit 204B in FIG. 2C can be switched off when it is not needed;however in some circumstances data is discarded by the initial reservecompression sub-unit 204A that would have been useful later and this mayreduce the accuracy of the decompressed data (for example, where data iscompressed initially from 8 bits to 6 bits and then from 6 bits to 4bits, the decompression from 4 bits back to 8 bits, may introduce moreerrors than if the data was compressed directly from 8 bits to 4 bits).

The methods described above with reference to FIGS. 2A-C may be used incombination with any compression threshold; however, in many examplesthe compression threshold will be 50% (although this may be expressed inanother way, such as 128 bytes for 256-byte data blocks). In exampleswhere a compression threshold other than 50% is used, the compressionthreshold may be selected to align with the burst size (e.g. 25%, 50% or75% for the example described above) and the architectures shown inFIGS. 2A-C provide the greatest efficiencies when this threshold can bemet using lossless compression for the majority of the data blocks(e.g. >50%) and lossy compression is only used for the remainder of theblocks.

To identify which compression technique was used (e.g. lossless orlossy), data may be appended that indicates the type of compression used(e.g. in a header) or this may be incorporated into any existing header(or header table) that is used or each compressed block of data mayinclude a number of bits, in addition to the compressed data, thatindicates the type of compression used (e.g. as described below withreference to FIGS. 3-11).

In any of the architectures of FIGS. 2A-C, there may be an additionalpre-processing step (not shown in FIGS. 2A-C) that is a lossypre-processing step and puts the source data into a suitable format forthe primary compression unit 202 and/or reserve compression unit 204,204A. This lossy pre-processing step may, for example, change the formatof the data from 10-bit (e.g. RGBA1010102) format into 8-bit (e.g.RGBA8883 or 8888 format) and two example methods for performing thispre-processing are described below with reference to FIGS. 14 and 16. Invarious examples, the method of FIG. 16 may be used as a pre-processingstep for the primary compression unit 202 and the method of FIG. 14 maybe used as a pre-processing step for the reserve compression unit 204,or vice versa, or the same method may be used for both the primary andreserve compression units.

The use of different data formats and/or pre-processing steps in thearchitectures of FIGS. 2A-C may also require modifications to thecompression methods used (e.g. in the primary compression unit 202and/or reserve compression units 204, 204A, 204B) and some examples ofthese are also described below. By combining a lossy pre-processing stepwith the lossless compression (implemented in the primary compressionunit 202), it will be appreciated that the compressed data which isoutput by the primary compression unit 202 is no longer lossless.

A lossy compression technique which guarantees that a pre-definedcompression threshold is met is described with reference to FIGS. 3-11.This technique may be implemented by the compression/decompression unit112 shown in FIG. 1 and/or by the reserve compression unit 204 shown inFIGS. 2A and 2B. As described above, use of data compression reduces therequirements for both memory storage space and memory bandwidth, andguaranteeing that a compression threshold (which may be defined in anysuitable way, as described above) is met ensures that benefits areachieved in terms of both memory storage space and memory bandwidth. Inthe examples described below the compression threshold is 50% (or 128bytes where each uncompressed data block is 256 bytes in size); howeverin other examples the method may be used for different compressionthresholds (e.g. 75% or 25%) and as described above the compressionthreshold selected may be chosen to correspond to an integer number ofbursts for memory transfer.

The lossy compression method shown in FIG. 3A takes as input, sourcedata in RGBA8888 or RGBX8888 format or in corresponding formats with thechannels in a different order (e.g. ARGB or other corresponding formatse.g. comprising four channels each having 8-bit values). The source datamay, in various examples, comprise channels with data values having lessthan 8 bits and examples of the consequential changes to the method aredescribed below (e.g. with reference to FIGS. 15A-D). In examples wherethe source data is not in a suitable format (e.g. where the RGB channelseach comprise more than 8-bits), a pre-processing step (e.g. asdescribed below with reference to FIG. 14 or FIG. 16) may be used toconvert the source data into an appropriate format. Alternatively, themethod of FIG. 3A may be used for data where the channels comprise morethan 8-bits (e.g. 10:10:10:2 data); however by using the pre-processingtechnique described below with reference to FIG. 14 which includes anHDR flag, there is one extra bit that can be shared across the RGBvalues. The following examples relate to compressing and decompressingimage data, e.g. in RGBA format, but it is to be understood that thesame principles can be applied for compressing and decompressing othertypes of data in other formats.

The source data that is input to the method of FIG. 3A comprises blocksof data. For image data each block of data relates to a tile (or block)of pixels (e.g. tiles comprising 8×8 pixels or 16×4 pixels) and eachblock is subdivided into a plurality of sub-blocks (block 302). Invarious examples, each block of data is subdivided (in block 302) intofour sub-blocks. If the block of data is subdivided (in block 302) intoa smaller number of larger blocks, then the amount of compression thancan be achieved may be larger but random access is made more difficultand unless many pixels in a block are accessed, the bandwidth usageincreases as the ‘data per accessed pixel’ would increase. Similarly,with a larger number of smaller blocks, random access is made easier(and the data per accessed pixels may be reduced); however the amount ofdata compression that can be achieved may be reduced. If, for example,the block of data relates to an 8×8 tile of pixels or a 16×4 tile ofpixels, the block may be subdivided into four sub-blocks 400 eachcorresponding to a 4×4 arrangement of pixels, as shown in FIG. 4A for an8×8 tile and FIG. 4B for a 16×4 tile. The sub-blocks may be denotedsub-block 0-3. Having performed this sub-division (in block 302), eachsub-block is considered independently and a lossy compression mode isselected for each sub-block based on the results of an analysis of thealpha values for the pixels within the sub-block (block 304). Dependentupon the outcome of this analysis, the selected mode may be a mode thatuses a constant value for alpha (as applied in block 306 and referred toas the constant alpha mode) or a mode that uses a variable value foralpha across the sub-block (as applied in block 308 and referred to asthe variable alpha mode). These may be the only two available modes oralternatively there may be one or more additional modes (e.g. as appliedin block 310). The compressed data for each sub-block (as output by oneof blocks 306-310) in a source data block is then packed together toform a corresponding compressed data block (block 312).

FIG. 4C shows a compressed data block 402 comprising compressed data 404for each of the sub-blocks and a further data field 406 that indicatesthat the lossy compression method of FIG. 3A is being used. The data 404for each sub-block 400 is divided into two fields: a 2-bit block mode408 and a 252-bit block data 410. The block mode bits 408 indicatewhether the variable alpha mode (block 308), constant alpha mode (block306), or other mode (block 310) is used. The field values may, forexample, be as follows:

Field value Interpretation 0b00 Constant alpha 0b01 Variable alpha 0b1-Other modes (where used)

An example implementation of the analysis stage (block 304) in FIG. 3Ais shown in detail in FIG. 5A. In this example, the alpha values foreach of the pixels within the sub-block are analysed and two parametersare computed: minalpha and maxalpha, which are the minimum and maximumvalues of alpha for all of the pixels in the sub-block (block 502).These may be determined in any way including, for example, use of a loop(as in the example pseudo-code below, or its functional equivalent) oruse of a tree of tests, with the first step determining maximum andminimum alpha values for pairs of pixels and then the second stepdetermining maximum and minimum alpha values for pairs of outputs fromthe first step, etc. These two parameters (minalpha and maxalpha) arethen used in a subsequent decision process (blocks 504-508) and althoughthe decision process is shown as being applied in a particular order, inother examples the same tests may be applied in a different order (e.g.blocks 506 and 508 may be swapped over, assuming alphadifftol<254).Furthermore, it will be appreciated that the test in block 504 mayalternatively be maxalpha>(minalpha+alphadifftol).

A first decision operation (block 504) assesses the range of alphavalues across the sub-block and determines whether the range is greaterthan the errors that would be introduced by the use of the (best case)variable alpha mode (in block 308). The size of these errors is denotedalphadifftol in FIG. 5A and this value may be predetermined. The valueof alphadifftol may be determined by comparing the loss in qualitycaused by the different methods within the variable alpha mode (i.e.4-colour encoding with 4 bits of alpha or 3-colour encoding with 5 bitsof alpha, and with two pixels sharing the same colour) in a trainingprocess (hence the use of the phrase ‘best case’ above). Alternatively,the value of alphadifftol may be determined (again in a trainingprocess) by assessing different candidate values against a large testset of images to find the candidate value that provides the best resultsusing either a visual comparison or an image difference metric. Thevalue of alphadifftol may be fixed or may be programmable.

In response to determining that the range is greater than the errorsthat would be introduced by the use of the (best case) variable alphamode (‘Yes’ in block 504), a variable alpha mode of compression (block308) is applied to this sub-block. However, in response to determiningthat the range is not greater than the errors that would be introducedby the use of the (best case) variable alpha mode (‘No’ in block 504), aconstant alpha mode of compression (block 306) is applied to thissub-block and two further decision operations (blocks 506, 508) are usedto determine the value of alpha which is used for the entire sub-block.If the value of maxalpha is the maximum possible value for alpha (e.g.0xFF, ‘Yes’ in block 506), then the value of alpha used in the constantalpha mode (constalphaval) is set to that maximum possible value (block510). This ensures that if there are any fully opaque pixels, they stayfully opaque after the data has been compressed and subsequentlydecompressed. If the value of minalpha is zero (e.g. 0x00, ‘Yes’ inblock 508), then the value of alpha used in the constant alpha mode(constalphaval) is set to zero (block 512). This ensures that if thereare any fully transparent pixels, they stay fully transparent after thedata has been compressed and subsequently decompressed. If neither ofthese conditions are held (‘No’ in both blocks 506 and 508), then anaverage value of alpha is calculated across the pixels in the sub-block(block 514) and used in the constant alpha mode.

The following pseudo-code (or its functional equivalent) may, forexample, be used to implement the analysis shown in FIG. 5 and in thiscode, P.alp is the alpha value for the pixel P being considered:

  CONST AlphaDiffTol = 4; U8 MinAlpha := 0xFF; U8 MaxAlpha := 0x00; U12AlphaSum := 0; FOREACH Pixel, P, in the 4x4block  MinAlpha := MIN(P.alp,MinAlpha);  MaxAlpha := MAX(P.alp, MaxAlpha);  AlphaSum += P.alp; ENDFORIF((MaxAlpha − MinAlpha) > AlphaDiffTol) THEN  Mode :=VariableAlphaMode; ELSEIF (MaxAlpha == 0xFF)  Mode := ConstAlphaMode; ConstAlphaVal := 0xFF; ELSEIF (MinAlpha == 0x00)  Mode :=ConstAlphaMode;  ConstAlphaVal := 0x00; ELSE  Mode := ConstAlphaMode; ConstAlphaVal := (AlphaSum + 8) >> 4; ENDIF

An alternative example implementation of the analysis stage (block 304in FIG. 3A) is shown in FIG. 5B. In this example, the parameterconstalphaval is set initially to the alpha value of a pixel at apre-defined location within the sub-block (block 520). For example,constalphaval may be set to the alpha value of the pixel at the top leftof the sub-block (i.e. the first pixel in the sub-block). All the alphavalues of the other pixels in the sub-block are then compared to thisconstalphaval (in blocks 522-526). Where all the alpha values are verysimilar to constalphaval (e.g. within a range of ±5, ‘Yes’ in block 522)then the constant alpha mode (of block 306 in FIG. 3A) is used, butwhere they vary more than this (‘No’ in block 522) then the variablealpha mode (of block 308 in FIG. 3A) is used. Then, in a similar mannerto the method of FIG. 5A, for the constant alpha mode, the parameterconstalphaval is set to zero (in block 512) or the maximum value (inblock 510) where the pixels are all nearly fully transparent(constalphaval<5, ‘Yes’ in block 524) or nearly fully opaque(constalphaval>250, ‘Yes’ in block 526) respectively. It will beappreciated that the particular values used in FIG. 5B as part of theanalysis (e.g. in blocks 522-526) are provided by way of example onlyand in other examples these values may differ slightly.

In comparison to the method of FIG. 5A, the method of FIG. 5B does notrequire the determination of minalpha and maxalpha which reduces thecomputational effort required to perform the analysis. However, themethod of FIG. 5B may produce some visible artefacts (e.g. aliasing)particularly when an object moves slowly across the screen and is lesslikely to detect a ‘constant alpha’ tile because of the use of apre-defined location as the centre of the alpha values.

Where the analysis of the alpha values within a sub-block (in block 304,e.g. as shown in FIG. 5A or 5B) determines that the constant alpha mode(of block 306) is to be used and also sets the value of the parameterconstalphaval (in one of blocks 510-514 and 520), then the compressionof the sub-block proceeds as shown in FIG. 6A or FIG. 7A. FIG. 6A showsa flow diagram of a first example method of compressing a sub-blockusing the constant alpha mode (block 306 of FIG. 3A). For each pixel,each of the RGB values are compressed from 8 bits to 5 bits (block 602e.g. as described below with reference to FIGS. 12A-B or using analternative truncation approach) and then the compressed values arepacked into a data block along with the value of constalphaval (block604). Therefore, in this example, the data for the 4×4 sub-block iscompressed from 512 bits (in RGBA8888 format (16*32=512 bits)) to 248bits (8+16*(5+5+5)=248 bits).

Two different ways of packing the compressed values into a data blockare shown in FIGS. 6B-6D, although it will be appreciated that thecompressed values may alternatively be packed into a data block in otherways. In the first example, as shown in FIG. 6B, the data block 606comprises three 80-bit fields 608-610, each comprising data from one ofthe three channels (e.g. R, G or B) and a 12-bit field 612 whichincludes the 8-bit constalphaval (e.g. as determined using FIG. 5A or5B) and the remaining 4 bits may be unused or reserved for future use.In each of the 80-bit fields 608-610 there are 5-bit values for each ofthe pixels in the sub-block.

In the second example, as shown in FIGS. 6C-6D the layout of data issimilar to that for the second method of performing the variable alphamode (of block 308, as described below with reference to FIG. 9) as thisresults in less complex hardware (and hence smaller hardware with alower power consumption) where the two methods (i.e. the methods ofFIGS. 6A and 9) are used together. As shown in FIG. 6C, each sub-block400 (e.g. each 4×4 pixel sub-block) is subdivided into four mini-blocks650-653 (e.g. four 2×2 pixel mini-blocks). Each mini-block has acorresponding 60-bit data field 660-663 that contains the RGB data forthe pixels in the mini-block. In FIG. 6C, the mini-blocks and theircorresponding data fields have been labelled Q, S, U and W. The 8-bitconstalphaval (e.g. as determined using FIG. 5A or 5B) is distributedamongst the four 3-bit fields 670-673. Within each of the mini-blockdata fields 660-663, the RGB data is distributed as shown in FIG. 6D. Ifeach of the mini-blocks comprises four pixels, labelled A-D, these areeach represented by three 5-bit values, one for each of the R, G and Bchannels (e.g. 5-bit values R_(A), G_(A) and B_(A) represent pixel A,5-bit values R_(B), G_(B) and B_(B) represent pixel B, etc.).

FIG. 7A shows a flow diagram of a second example method of compressing asub-block using the constant alpha mode (block 306 of FIG. 3A) which isa variant of the method shown in FIG. 6A. As shown in FIG. 7A, thepixels in the sub-block 400 are divided into two non-overlapping subsetsand then the pixels in each of the subsets are compressed by differentamounts. In the specific example shown in FIG. 7A which may be usedwhere the constalphaval can be stored in less than 8 bits (e.g. theconstant alpha is <5 or >250), the pixels in the first subset arecompressed in the same way as in FIG. 6A, i.e. by converting each of theR, G, B values from RGB888 format to RGB555 format (block 602), whereasthe pixels in the second subset are compressed in a different way (block702), i.e. by converting the RGB data from RGB888 format (i.e. three8-bit values, one for each channel) to RGB565 format (i.e. 5-bit valuesfor the R and B channels and 6-bit values for the G channel). Thecompression (in blocks 602 and 702) may be performed as described belowwith reference to FIGS. 12A-B or may use an alternative approach (e.g.truncation of the values by removing one or more of the LSBs). In otherexamples, the two subsets of pixels may be compressed in different ways(e.g. the pixels of the second subset may be compressed by convertingthe RGB data from RGB888 format to RGB554).

FIG. 7B shows an example of how the pixels in a sub-block may be dividedinto the two subsets. In the example shown in FIG. 7B, the 4 pixelsmarked A-D form the first subset and the 12 shaded pixels form thesecond subset. In other examples, the split between the two subsets maybe different (e.g. there may more or fewer than four pixels in the firstsubset, with the remaining pixels forming the second subset) and/or theposition of the pixels in the first subset may be different.

In examples where the constalphaval is an 8-bit value (e.g. where theconstant alpha is not <5 or >250), the method of FIG. 7A may be modifiedsuch that the pixels in the second subset are compressed in the same wayas in FIG. 6A, i.e. by converting each of the R, G, B values from RGB888format to RGB555 format (block 602), whereas the pixels in the firstsubset are compressed in a different way (block 702), i.e. by convertingthe RGB data from RGB888 format (i.e. three 8-bit values, one for eachchannel) to RGB565 format (i.e. 5-bit values for the R and B channelsand 6-bit values for the G channel). As before, the compression (inblocks 602 and 702) may be performed as described below with referenceto FIGS. 12A-B or may use an alternative approach (e.g. truncation ofthe values by removing one or more of the LSBs).

Where the analysis of the alpha values within a sub-block (in block 304,e.g. as shown in FIG. 5A or 5B) determines that the variable alpha mode(of block 308) is to be used, then the compression of the sub-blockproceeds as shown in FIG. 8A or 9. FIG. 8A shows a flow diagram of afirst example method of compressing a sub-block using the variable alphamode (block 308 of FIG. 3A). As shown in FIG. 8A, the data for eachpixel in the sub-block is compressed by converting from RGBA8888 format,i.e. four 8-bit values, one for each channel including the alphachannel, to RGBA5555 format, i.e. four 5-bit values, one for eachchannel including the alpha channel, (block 802, e.g. as described belowwith reference to FIGS. 12A-B). The pixels in the sub-block 400 are thendivided into two non-overlapping subsets (e.g. as described above withreference to FIG. 7B) and the pixels in the first subset (e.g. pixelsA-D in FIG. 7B) are then subject to further compression (blocks 804-806)which can be described with reference to FIG. 8B. To further compressthe pixels in the first subset, each of these pixels is compared to itsneighbour pixels to identify which neighbouring pixel is most similar(block 806). The similarity may be assessed using a colour differenceparameter, with the most similar pixel having the smallest colourdifference to the particular pixel and where colour difference between apixel and a neighbour pixel (i.e. between a pair of pixels) may becalculated as:|Red difference|+|Green difference|+|Blue difference|+|Alphadifference|  (1)

Having identified the most similar neighbouring pixel for each pixel inthe first subset, an index is selected for each pixel in the firstsubset using a look-up table, such as:

Index Most similar neighbouring pixel 000 R0 001 R1 010 R2 011 R3 100 R4Where the references for the neighbouring pixels R0-R4 are defined asshown in FIG. 8B. It will be appreciated however, that where thepositions of the pixels of the first subset are different, these may bedefined differently.

In various examples, there may be additional indices that are used wherethere is a gradient around a pixel in the first subset. In suchexamples, the pixel data for one or more additional notional neighbourpixels are calculated, for example by the addition of a singleadditional notional neighbour pixel, R5, where the pixel data for R5 iscalculated using:R5=(R0+R1)/2In other examples, one or more further notional neighbour pixels mayalso be considered, e.g. R6 and/or R7, where the pixel data for R6 andR7 is calculated using:R6=(R0+R4)/2R7=(R1+R3)/2

Where additional notional neighbour pixels are used, the look-up tableincludes corresponding additional entries to identify the indices, forexample:

Index Most similar neighbouring pixel 000 R0 001 R1 010 R2 011 R3 100 R4101 R5 110 R6 111 R7

In various examples, there may be an additional special case index thatindicates that none of the neighbouring pixels (including any notionalneighbouring pixels, where used) are sufficiently similar to theparticular pixel. This may, for example, be determined based on athreshold and where the closest colour difference exceeds thisthreshold, an index of 000 may be used. In an example, the threshold maybe 31 for 5-bit values. In addition to using the index 000, the pixelreferred to by the index 000 is changed to a value that is an averagebetween the current pixel and the pixel referred to. Alternatively, in avariation of FIG. 8A, if the conversion of pixel data from RGBA8888 toRGBA5555 (in block 802) is not performed until immediately prior to thepacking of data values (in block 808), such that the comparison (inblock 804) is performed on 8-bit values, the threshold will be different(e.g. 255 for 8-bit values).

FIG. 9 shows a flow diagram of a second example method of compressing asub-block using the variable alpha mode (block 308 of FIG. 3A). As shownin FIG. 9, the sub-block 400 (e.g. each 4×4 pixel sub-block) issubdivided (block 902) into four mini-blocks 650-653 (e.g. 2×2 pixelmini-blocks), as shown in FIG. 6C. Each mini-block is then compressedindividually, with each mini-block having a corresponding data field660-663 that contains the RGB data for the pixels in the mini-block. Thecorresponding four 3-bit fields 670-673 do not contain theconstalphaval, as is the case in the earlier discussion of FIG. 6C, butwhere variable alpha mode is used, these 3-bit fields identify theencoding (or palette) mode that is used for each of the mini-blocks inthe sub-block, as described below.

The encoding mode is determined for each of the mini-blocks based oncolour differences that are calculated for each pixel pair in themini-block (block 904). The colour difference may be calculated usingequation (1) above and this may be implemented by the functionalequivalent of the pseudo-code provided below in which the colourdifference is clamped to 6 bits (i.e. a maximum value of 63). In thiscode, the notation is as follows: IntermediateResult[5,0] refers to the6 LSBs of the IntermediateResult 10-bit value and red/grn/blu/alp referto red/green/blue/alpha respectively).

U6 DiffMetric(PIXEL Pix1, PIXEL Pix2) {  U6 Result;  U10IntermediateResult;  U8 R1 := Pix1.red;  U8 R2 := Pix2.red;  U8 G1 :=Pix1.grn;  U8 G2 := Pix2.grn;  U8 B1 := Pix1.blu;  U8 B2 := Pix2.blu; U8 A1 := Pix1.alp;  U8 A2 := Pix2.alp;  IntermediateResult:= SAD4x5(R1,R2, G1, G2, B1, B2, A1, A2);  IF((IntermediateResult > 63) THEN   Result:= 63;  ELSE   Result := IntermediateResult[5..0];  ENDIF  RETURNResult; }

The pseudo-code above includes a sum of absolute differences (SAD)function and this may, for example, be implemented in any way (e.g. asimplemented by a logic synthesis tool or as described in FIG. 2 of“Efficient Sum of Absolute Difference Computation on FPGAs” by Kumm etal).

Having calculated the colour differences (in block 904), the smallestcolour difference for any pixel pair in the mini-block is used todetermine the mini-block encoding mode that is used (block 906). Thereare two distinct types of mini-block encoding mode that are useddependent upon whether the smallest colour difference (between any pixelpair in the mini-block) exceeds a threshold value (which may, forexample be set at a value in the range 0-50, e.g. 40). If the smallestcolour difference does not exceed the threshold (‘Yes’ in block 906),then one of a plurality of encoding patterns are used (as selected inblock 908) and three per-mini-block palette colours are stored (blocks910-914). However, if the smallest colour difference does exceed thethreshold (‘No’ in block 906) then a four colour mode is used (block916). These different mini-block modes are described in detail below.The encoding patterns rely on an assumption that in the majority ofmini-blocks there are no more than three distinct colours and in suchcases the mini-block can be represented by three palette colours alongwith an assignment of pixels to palette entries. The four colour mode ispresent to handle the exceptions to this.

As noted above, the value of the threshold may be in the range 0-50. Invarious examples it may be a fixed value that is set at design time.Alternatively, it may be a variable which is stored in a global registerand read each time the method of FIG. 9 is performed. This enables thethreshold to be changed dynamically or at least periodically. In variousexamples, the value of the threshold may be set based on results from atraining phase. The training phase may use an image quality metric, forexample peak signal-to-noise ratio (PSNR) or structural similaritymetric (SSIM) to assess a selection of images compressed using each ofthe three colour approach (blocks 908-912) or the four colour approach(block 916) and then the threshold value may be selected such thatoverall the highest image quality metrics are obtained.

It may be noted that the threshold used in the method of FIG. 9 may havea different value to the threshold used in the method of FIG. 8A becausethe two thresholds serve different purposes as they are aiming toaddress different situations and remove different kinds of artefact. Inthe method of FIG. 8A, the threshold is used to identify when one of thepixels is an ‘isolated colour’, in that it cannot be represented wellwith one of its neighbours. In contrast, the threshold in the method ofFIG. 9, the threshold is used to identify when the four colours are toodifferent to each other.

As shown in FIG. 9, if the smallest colour difference is smaller than orequal to the threshold (‘Yes’ in block 906), such that an encodingpattern can be used, the particular pattern that is used is selectedfrom a set of six assignment patterns as shown in FIG. 10A based on thepixel pair (in the mini-block) that has the smallest colour difference(block 908). In each pattern in FIG. 10A, the two pixels that are shownshaded share a palette entry that is derived by averaging the two sourcepixel colours. Determining the three palette colours P₀, P₁, P₂ (block910) for the selected pattern (from block 908) therefore comprisesperforming this average and identifying the pixel data for the other tworemaining pixels in the mini-block. Example pseudo-code for implementingthis selection along with the calculation of three palette colours isprovided below, but referring to FIG. 10A, if the smallest colourdifference is for pixel pair AB (denoted DiffAB in the pseudo-code),then the mode that is selected is the mode called ‘Top’, and the threepalette colours are an average of A and B, and then pixels C and D.Similarly, if the smallest colour difference is for pixel pair CD(denoted DiffCD in the pseudo-code), then the mode that is selected isthe mode called ‘Bottom’ and the three palette colours are an average ofC and D and then pixels A and B. The palette colours in other modes(i.e. the ‘Left’, ‘Right’, Diag1’ and ‘Diag2’ modes) are apparent fromFIG. 10A.

The following table shows an example mapping of the correspondingpalette colours for each of the pixels A-D in a mini-block for each ofthe encoding modes along with the encoding value that is stored in the3-bit field 670-673 for the mini-block (as shown in FIG. 6C). The pixelsthat are represented by identical palette colours are shown in bold anditalic. In this example, the mapping has been arranged so that eachpixel (e.g. pixel A, B, C or D) accesses only one of two possiblepalette colours and this results in a hardware implementation which isless complex than if all four pixels can access any of the three palettecolours.

Encoding Pattern Pixel A Pixel B Pixel C Pixel D value name (P₀ or P₁)(P₀ or P₁) (P₀ or P₂) (P₀ or P₂) 000 Top

P₀ P₂ 001 Bottom P₀ P₁

010 Left

P₁

P₂ 011 Right P₁

P₂

100 Diag 1

P₁ P₂

101 Diag 2 P₁

P₂

A portion of example pseudo-code that implements this table (and blocks908-910 of FIG. 9) is as follows:

  ELSIF (MinDiff == DiffAB)  MODE := TOP;  P0full:= PixelC;  P2full:=PixelD;  P1full:= Average(PixelA, PixelB); ELSIF (MinDiff == DiffAC) MODE := LEFT;  P0full:= Average(PixelA, PixelC);  P1full:= PixelB; P2full:= PixelD; ELSIF . . .

The Average function in the pseudo-code above may be implemented asfollows (or its functional equivalent):

  PIXEL Average(PIXEL Pix1, PIXEL Pix2) {  PIXEL Result;  Result.red :=(Pix1.red + Pix2.red + 1) >> 1;  Result.grn := (Pix1.grn +Pix2.grn + 1) >> 1;  Result.blu := (Pix1.blu + Pix2.blu + 1) >> 1; Result.alp := (Pix1.alp + Pix2.alp + 1) >> 1;  Return Result; }

Having determined the three palette colours, P₀, P₁, P₂ in RGBA8888format, the values are converted to RGBA5555 format (block 912, e.g. asdescribed below with reference to FIGS. 12A-B). Whilst the operations ofgenerating the palette colours (in block 910) and the compression byconverting format (in block 912) are shown and described separately, itwill be appreciated that the two operations may be combined into asingle step and this may result in less complex hardware.

As shown in FIG. 9, if the smallest colour difference is not smallerthan or equal to the threshold (‘No’ in block 906, i.e. the threshold isexceeded), then the encoding patterns shown in FIG. 10A cannot be usedand instead the four colour mode is used. In this mode, the number ofbits used to represent each channel is reduced (in block 916) and in theexample shown in FIG. 9, the pixel data is converted from RGBA8888format to RGBA4434 format for each of the four pixels in the mini-block(block 916 e.g. as described below with reference to FIGS. 12A-B). Theencoding value that is used to indicate that this mode is being used isone of the values that are not used for the encoding patterns (asdetailed in the table above) and in various examples an encoding valueof 111 may be used. This encoding value is stored in the 3-bit field670-673 for the mini-block (as shown in FIG. 6C).

Whilst the four colour mode is shown as using RGBA4434 format,alternative compressed formats may alternatively be used (e.g.RGBA4443). The RGBA4434 format may be used because although the humaneye is sensitive to blue, the human visual system does not perceive thecolour with the same resolution as red or green. In addition, a ditherpattern may additionally be applied when using the four colour mode(e.g. in block 916 and also block 602 in FIG. 11).

Having generated the compressed data for a mini-block, using any one ofthe encoding patterns or the four colour mode, the compressed data forthe mini-block is packed into a 60-bit data field 660-663 for themini-block and the corresponding encoding value is inserted in theassociated field 670-673 (block 914). The way that the data is packeddiffers depending upon the mode used and these are shown in FIGS. 10Band 10C. FIG. 10B shows the packing of pixel data for a mini-block thathas been compressed using an encoding pattern and FIG. 100 shows thepacking of pixel data for a mini-block that has been compressed usingthe four colour mode.

As described above, for a mini-block that has been compressed using anencoding pattern, the data to include in the data field comprises threepalette colours in RGBA5555 format and these may be packed into the datafield as shown in FIG. 10B. As shown, the RGB data may be separated fromthe alpha channel data in order to more closely match the packingarrangement that may be used in the constant alpha mode (as shown inFIG. 6D). For a mini-block that has been compressed using the fourcolour mode, the data to include in the data field comprises fourRGBA4434 values and these may be packed into the data field as shown inFIG. 100. Again the arrangement shown is selected to more closely matchthe packing arrangement that may be used in the constant alpha mode. Thearrangement for packing the data for each pixel is also shown in FIG.100 and by packing the data in this way, with the four alpha bits beingseparated and placed between the RGB data, the R, G, B channel datacorresponds to the positions of, respectively, four, four and three MSBsof the RGB data of the constant alpha mode shown in FIG. 6D. The alphachannel for the given pixel is distributed in the unused sections,matching LSB to MSB order.

Having formed the data fields for the mini-blocks (in block 914), theseare then packed into data fields to the sub-blocks (block 918, e.g. asshown in FIG. 6C).

Although the encoding patterns are described above in relation to thevariable alpha mode, in various examples they may also be used toprovide a higher-precision constant alpha mode where the constant alphavalue is 255 (or other pre-defined value) as shown in FIG. 11, with thepreviously described constant alpha mode (e.g. as shown in FIG. 6A or7A) being used for other constant alpha values. FIG. 11 shows a flowdiagram of a third example method of compressing a sub-block using theconstant alpha mode (block 306 of FIG. 3A) which is a variant of themethods shown in FIGS. 6A and 9. In this example method, the sub-block400 (e.g. each 4×4 pixel sub-block) is subdivided (block 902) into fourmini-blocks 650-653 (e.g. 2×2 pixel mini-blocks), as shown in FIG. 6C.Each mini-block is then compressed individually, with each mini-blockhaving a corresponding data field 660-663 that contains the RGB data forthe pixels in the mini-block. The corresponding four 3-bit fields670-673 identify the encoding mode that is used for each of themini-blocks in the sub-block. In this example, the constalphaval is notincluded as it is a known constant value, e.g. 255, and this in turn maybe indicated using a dedicated block encoding mode (e.g. one of the0b1—modes which are indicated as reserved above). In yet furtherexamples, the encoding patterns may in addition, or instead, be used toprovide higher-precision constant alpha mode where the constant alphavalue is stored and not assumed to have a known constant value (e.g.255). In such examples, constalphaval may be stored alongside the pixeldata and in order that there is room to store constalphaval, the methodof FIG. 11 may be modified such that the three palette colours areconverted from RGB888 to RGB676 (in block 1112, instead of RGB776) andthe four-colour mode converts each colour from RGB888 to RGB554 (inblock 602, instead of RGB555).

The encoding mode is then determined for each of the mini-blocks basedon colour differences that are calculated for each pixel pair in themini-block (block 904). The colour difference may be calculated usingany of the methods described above but omitting the alpha channel data.

Having calculated the colour differences (in block 904), the smallestcolour difference for any pixel pair in the mini-block is used todetermine the mini-block encoding mode that is used (block 906). Thereare two distinct types of mini-block encoding mode that are useddependent upon whether the smallest colour difference (between any pixelpair in the mini-block) exceeds a threshold value (which may, forexample be set at a value in the range 0-50, e.g. 40). If the smallestcolour difference does not exceed the threshold (‘Yes’ in block 906),then one of a plurality of encoding patterns are used (as selected inblock 908) and three per-mini-block palette colours are stored (blocks910-914). However, if the smallest colour difference does exceed thethreshold (‘No’ in block 906) then the earlier compression approach(from FIG. 6A), of converting each of the RGB values from 8 bits to 5bits is used (block 602). As before the encoding patterns rely on anassumption that in the majority of mini-blocks there are no more thanthree distinct colours and in such cases the mini-block can berepresented by three palette colours along with an assignment of pixelsto palette entries. The earlier approach (of block 602) present tohandle the exceptions to this. Where encoding patterns are used, higherprecision can be used (e.g. more than 5 bits per colour channel) asthere are only three palette colours rather than four pixels.

The selection of an encoding pattern then proceeds as described abovewith reference to FIG. 9, except that there is no alpha data to consider(blocks 908-910). Having determined the three palette colours, P₀, P₁,P₂ in RGB888 format, the values are converted to a format with higherresolution than RGB555, e.g. RGB776 or RGB666 format (block 1112, e.g.by truncation or as described below with reference to FIGS. 12A-B).

Having generated the compressed data for a mini-block, using either oneof the encoding patterns or the four colour mode (although in someexamples, this four colour mode may not be used where alpha is constantvalue of 255), the compressed data for the mini-block is packed into a60-bit data field 660-663 for the mini-block and the correspondingencoding value is inserted in the associated field 670-673 (block 1114).As described above, in various examples where the encoding patterns areused to provide higher-precision constant alpha mode, constalphaval mayalso be packed into the 60-bit data fields 660-663. Having formed thedata fields for the mini-blocks (in block 1114), these are then packedinto data fields to the sub-blocks (block 1118, e.g. as shown in FIG.6C). This method of FIG. 11 provides improved RGB accuracy (e.g. RGB776or RGB676 instead of RGB555) and hence improved image quality.

In the description above, there are six different palette assignment (orencoding) modes, as shown in FIG. 10A (along with a four colour mode).In various examples there may be one or more additional mode, as shownin FIG. 10D. In such an additional mode there are only two palettecolours (as indicated by the different shadings in FIG. 10D), with eachof the two palette colour being the average of the two pixels sharingthe same palette colour. In the first example, 1001, pixels A and C areaveraged to generate the first palette colour, which is then used toreplace the data for pixels A and C and similarly for pixels B and D togenerate the second palette colour (which is then used to replace thedata for those pixels). In the second example, 1002, pixels A and B areaveraged to generate the first palette colour and pixels C and D areaveraged to generate the second palette colour. As only two palettecolours are stored (compared to the three in the earlier encodingmodes), the palette colours may be stored at higher precision (e.g.RGBA8888 for constant alpha and RGBA7878 for variable alpha).

The first of the two palette colour modes 1001 (as shown in FIG. 10D)may, for example be used if the colour difference between pixels A and C(DiffAC in the notation used above) and the colour difference betweenpixels B and D (DiffBD) are both less than a pre-defined threshold.Similarly, the second of the two palette colour modes 1002 may, forexample be used if the colour difference between pixels A and B (DiffABin the notation used above) and the colour difference between pixels Cand D (DiffCD) are both less than a pre-defined threshold. Furthermore,whilst FIG. 10D shows only two palette colour modes, it will beappreciated that there may, in various examples, be a third two palettecolour modes in which one palette colour is an average of pixels A and Dand the other palette colour is an average of pixels B and C.

Where the two palette colour modes are used, an additional test againstthe pre-defined threshold (which may be different from the thresholdused in block 906) may be inserted between blocks 904 and 906 in FIG. 9.If the test determines that a two palette colour mode may be used (asdescribed above), the pixel data is averaged to form the two palettecolours and then those palette colours may be converted to RGB7878format (or they may be left as RGB888 for the constant alpha case) andthen the pixel data is packed into a data field (block 914). If the testdetermines that a two palette colour mode cannot be used, then themethod proceeds to block 906.

As detailed above, there are many operations in data compression thatinvolve converting an n-bit number, N, to an m-bit number, M, where n>m.The ideal mapping from n to m bits is given by:

$M = {{{IdealMap}( {N,n,m} )} = {{floor}\mspace{20mu}( {{\frac{( {2^{m} - 1} )}{( {2^{n} - 1} )}N} + \frac{1}{2}} )}}$

This may, for example, be implemented using a large look-up table (LUT)comprising N entries. Alternatively, since the results are ‘symmetrical’around a half, in that:IdealMap( N,n,m)=IdealMap(N,n,m)where X means the complement of the bits of X, then the size of the LUTmay be halved. In such examples, one half of the LUT may be retained(e.g. the lower half) and the other half of the table (e.g. the upperhalf) may be derived by inverting N−1 LSBs on input and similarly,inverting M−1 LSBs on output (e.g. by XORing X-1 LSBs with the MSB—theMSB is the same on input and output).

FIG. 12A is a flow diagram of an alternative method of performing thisconversion. Using the method of FIG. 12A provides a hardwareimplementation, as shown in FIG. 12B, that is both accurate andefficient (e.g. in hardware, i.e. in terms of size, etc.). Whilst itstill uses a LUT, the LUT is significantly smaller than the LUTsmentioned above (but does require an additional addition element).Whilst the method is described herein in the context of datacompression, the methods described herein with reference to FIG. 12A mayalso be used in other situations and independently of any other methodsdescribed herein.

As shown in FIG. 12A, the method comprises receiving an input n-bitnumber, N, and truncating that number from n-bits to m-bits (block 1202)and this may be implemented in the truncation hardware unit 1222 in FIG.12B. An adjustment value is then determined (block 1204) based on theinput n-bit number, N, and, as described below, this may be implementedusing a number of AND and OR gates in the look-up logic unit 1224. TheseAND and OR logic gates (or alternative logic arrangements that arefunctionally equivalent) compare a plurality of pre-determined subsetsof the bits of the input n-bit number with pre-determined values infixed-function circuitry and based on the outcome of the comparisons,determine an adjustment value which is then added to the truncated value(from block 1202 and truncation hardware unit 1222) in theincrement/decrement unit 1226 (block 1206). The value of the adjustmentvalue is either zero, one, or minus one.

The operation of the look-up logic unit 1224 to determine the adjustmentvalue (in block 1204) can be described with reference to a specificexample where n=8 and m=5 and the following VHDL (or its functionalequivalent), where “0-0-111” is an example of a pre-defined bit sequencewhich is compared to the bits in the input n-bit number and “01” itsassociated adjustment value:

function CorrectionsFor8to5(i : std_logic_vector(7 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00---11-”) thenresults:= “01”; end if;  if std_match(i, “0-0--111”) then results:=results OR “01”; end if;  if std_match(i, “000--1-1”) then results:=results OR “01”; end if;  if std_match(i, “0-00011-”) then results:=results OR “01”; end if;  if std_match(i, “0--00111”) then results:=results OR “01”; end if;  if std_match(i, “1--11000”) then results:=results OR “11”; end if;  if std_match(i, “111--0-0”) then results:=results OR “11”; end if;  if std_match(i, “1-11100-”) then results:=results OR “11”; end if;  if std_match(i, “11---00-”) then results:=results OR “11”; end if;  if std_match(i, “1-1--000”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor8to5;

In this example, ‘std_logic_vector(7 downto 0)’ refers to all 8 bits ofN (labelled 0-7, with bit 0 being the LSB and bit 8 being the MSB) andthe function ‘std_match’ may be implemented using AND gates, forexample, ‘std_match(i, “00---11-”)’ may be implemented as follows:b7∧ b6∧b2∧b1Where by is bit y of N and ∧ represents a logical AND. Similarly‘std_match(i, “11---00-”)’ may be implemented as:b7∧b6∧ b2∧ b1Furthermore, in this example, the initial value of ‘results’ is set tothe two LSBs (i.e. bits 0 and 1) of N and the final value of ‘results’is either set to 01 or 00 or to the initial value of results combinedusing an OR gate with a 2-bit value that is dependent upon the outcomeof the ‘std_match’ function described above. This assumes that commonfactors are shared. It is estimated that approximately 31 AND/OR gatesare required to implement the look-up logic unit 1224 for n=8 and m=5;however, it will be appreciated that any equivalent logic functionality(e.g. implemented using NANDs, NORs, XORs, etc.) may alternatively beused, e.g. dependent upon the silicon process used for fabrication ortarget clock speed).

In this way, the look-up logic unit 1224 compares pre-determined subsetsof the bits of the input n-bit number with pre-determined values infixed-function circuitry, and sets the adjustment value in dependence onthe results of the comparisons. Implementing the conversion from ann-bit number to an m-bit number in hardware using AND/OR gates asdescribed above to determine an adjustment value to be applied to atruncated version of the n-bit number provides a very efficientimplementation (e.g. in terms of silicon area, latency and powerconsumption) for performing a particular conversion (e.g. 8-bit valuesto 5-bit values). In this example, the hardware is inflexible in thesense that it is implemented in fixed-function circuitry using AND andOR gates for performing one or more specific conversions (e.g.conversions from 8-bit numbers to 5-bit numbers). If differentconversions are required, e.g. different values of n and/or m, thendifferent hardware can be used. The trade-off for this inflexibility isthat the hardware can be implemented very efficiently (in terms ofsilicon area, latency and power consumption). Since the compressionprocess described herein uses an 8-bit to 5-bit converter, and this isknown at design time, the inflexibility is not a big problem whereas theefficiency advantages can be significant.

Although data decompression is not described above, it will beappreciated that the data compression operations described above arereversed in order to achieve data decompression. For example, themethods of FIGS. 3A, 6A, 7A, 8A, 9 and 11 are reversed when performingdata decompression. FIG. 3B shows a method of data decompression whichis the reverse of the data compression method shown in FIG. 3A. As shownin FIG. 3B, for each sub-block of the compressed data, the compressionmode is identified from the block mode (block 322) and depending uponthe mode, the compressed sub-block data is decompressed using a variablealpha mode (block 308′, which is the reverse of block 308 describedabove), or a constant alpha mode (block 306′, which is the reverse ofblock 306 described above) and similarly if any other data compressionmode is used (block 310′). The data decompression of the sub-blocks (inblocks 306′, 308′, 310′) may involve converting a number from n-bits tom-bits where n<m (e.g. in inverse operations to blocks 602, 702, 802,912, 916 and 1112). This may be implemented using the method shown inFIG. 13A and the hardware shown in FIG. 13B.

As shown in FIG. 13A, the method comprises receiving an input n-bitnumber, N, and using bit replication to expand that number from n-bitsto m-bits (block 1302) and this may be implemented in the replicationhardware unit 1322 in FIG. 13B. This bit replication involves appendingthe (m−n) MSBs from N to the least significant end of N to generate anintermediate m-bit number, R, as shown in FIG. 13C. If m>2n, then(m−n)>n and hence it is necessary to first append all n-bits (i.e. theentirety of N) at least once (e.g. k times) and then append the (m−kn−n)MSBs from N to form the intermediate m-bit number R, as shown in FIG.13D. The bit replication operation may alternatively be described asfollows: assuming (n≤m) and defining k=└(m/n)┘ and r=m mod n, form a newvalue, R, comprising, in the MSB position, k repetitions of N, followed,in LSBs, the r MSBs of N.

An adjustment value is then determined (block 1204) based on the inputn-bit number, N, and, as described below, this may be implemented usinga number of AND and OR gates in the look-up logic unit 1224. These ANDand OR logic gates (or alternative logic arrangement that isfunctionally equivalent) compare a plurality of pre-determined subsetsof the bits of N with pre-determined values in fixed-function circuitryand based on the outcome of the comparisons, determine an adjustmentvalue which is then added to the replicated value (from block 1302 andreplication hardware unit 1322) in the increment/decrement unit 1226(block 1306). The value of the adjustment value is either zero, one, orminus one. In some examples, i.e. for some combinations of values of nand m (where n<m), the adjustment value is always zero and hence boththe increment/decrement unit 1226 and the look-up logic unit 1224 can beomitted from FIG. 13B.

The operation of the look-up logic unit 1224 to determine the adjustmentvalue (in block 1204) can be described with reference to a specificexample where n=5 and m=8 and the following VHDL (or its functionalequivalent):

  function CorrectionsFor5to8(i : std_logic_vector(4 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00-11”) then results:=“01”; end if;  if std_match(i, “11-00”) then results:= results OR “11”;end if;  return results; end function CorrectionsFor5to8;

In this example, ‘std_logic_vector(4 downto 0)’ refers to all 5 bits ofN (labelled 0-4, with bit 0 being the LSB and bit 4 being the MSB) andthe function ‘std_match’ may be implemented using AND gates, forexample, ‘std_match(i, “00-11”)’ may be implemented as follows:b4∧ b3∧b1∧b0Where by is bit y of N and ∧ represents a logical AND. Similarly‘std_match(i, “11-00”)’ may be implemented as:b4∧b3∧ b1∧ b0Furthermore, in this example, the initial value of ‘results’ is set tothe two LSBs (i.e. bits 0 and 1) of N and the final value of ‘results’is either set to 01 or 00 or to the initial value of results combinedusing an OR gate with 11. It is estimated that approximately 7 AND/ORgates are required to implement the look-up logic unit 1224 for n=5 andm=8.

The look-up logic unit 1224 compares pre-determined subsets of the bitsof the input n-bit number with pre-determined values in fixed-functioncircuitry, and sets the adjustment value in dependence on the results ofthe comparisons. As described above, the use of fixed-function hardwareusing the AND and OR gates described above allows the hardware to beimplemented very efficiently in terms of silicon area, latency and powerconsumption. The hardware may be configured to perform conversions for asmall group of different values for n and m.

Further examples below show how the adjustment value is calculated (inblock 1204 and unit 1224) for other values of n and m.

For n=8 and m=4, the following example VHDL shows how the adjustmentvalue is calculated (in block 1204 and unit 1224):

function CorrectionsFor8to4(i : std_logic_vector(7 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00--11--”) thenresults:= “01”; end if;  if std_match(i, “000-1-1-”) then results:=results OR “01”; end if;  if std_match(i, “0-0-111-”) then results:=results OR “01”; end if;  if std_match(i, “00001--1”) then results:=results OR “01”; end if;  if std_match(i, “0-0011-1”) then results:=results OR “01”; end if;  if std_match(i, “00-01-11”) then results:=results OR “01”; end if;  if std_match(i, “0--01111”) then results:=results OR “01”; end if;  if std_match(i, “1--10000”) then results:=results OR “11”; end if;  if std_match(i, “11-10-00”) then results:=results OR “11”; end if;  if std_match(i, “1-1100-0”) then results:=results OR “11”; end if;  if std_match(i, “11110--0”) then results:=results OR “11”; end if;  if std_match(i, “1-1-000-”) then results:=results OR “11”; end if;  if std_match(i, “111-0-0-”) then results:=results OR “11”; end if;  if std_match(i, “11--00--”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor8to4;It is estimated that approximately 41 AND/OR gates are required toimplement the look-up logic unit 1224 for n=8 and m=4. In contrast, forn=4 and m=8 only bit replication is required and the adjustment value iszero in all cases.

For n=8 and m=3, the following example VHDL shows how the adjustmentvalue is calculated (in block 1204 and unit 1224):

function CorrectionsFor8to3(i : std_logic_vector(7 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00-11---”) thenresults:= “01”; end if;  if std_match(i, “0001-1--”) then results:=results OR “01”; end if;  if std_match(i, “0-0111--”) then results:=results OR “01”; end if;  if std_match(i, “0001--11”) then results:=results OR “01”; end if;  if std_match(i, “00-1-111”) then results:=results OR “01”; end if;  if std_match(i, “11-0-000”) then results:=results OR “11”; end if;  if std_match(i, “1110--00”) then results:=results OR “11”; end if;  if std_match(i, “1-1000--”) then results:=results OR “11”; end if;  if std_match(i, “11-00---”) then results:=results OR “11”; end if;  if std_match(i, “1110-0--”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor8to3;It is estimated that approximately 31 AND/OR gates are required toimplement the look-up logic unit 1224 for n=8 and m=3. In contrast, forn=3 and m=8 only bit replication is required and the adjustment value iszero in all cases.

The compression methods described above with reference to FIGS. 2A-C, 3Aand 4-11 take as input, source data in RGBA8888 or RGBX8888 format or incorresponding formats with the channels in a different order (e.g.ARGB). As mentioned above, in examples where the source data is not inthis format, a pre-processing step may be used to convert the sourcedata into an appropriate format (e.g. into 888Z where Z is an integerthat is not greater than 8). A first example of this pre-processing stepcan be described with reference to FIG. 14 that converts data valuesfrom RGBA10:10:10:2 format to RGBA8883. An alternative technique forpre-processing 10-bit data is described below with reference to FIG. 16.By using one or both of these pre-processing methods, the likelihoodthat the compression threshold is met (and hence a lossless compressionmethod in the primary compression unit 202 of any of FIGS. 2A-C is used)is increased; although minor modifications are required to the methodsperformed by the architectures shown in FIGS. 2A-C, as described below.Furthermore, whilst these conversion methods are described herein interms of a pre-processing step for the compression methods describedabove, both of the conversion methods may alternatively be usedindependently of any of the other methods described herein. It willfurther be appreciated that whilst the compression method implemented bythe primary compression unit 202 is itself lossless, if used incombination with the conversion method of FIG. 14 (which is notlossless), the overall data compression method is lossy. The conversionmethod of FIG. 16 is lossless and so is suited to be used as apre-processing step for the primary compression unit 202 as the overallmethod is still lossless; however, if having performed the method ofFIG. 16, any bits from the fourth pseudo-channel are discarded, then theoverall data compression method becomes lossy.

FIG. 14 shows a flow diagram of a first example method of convertingpixel data from RGBA10:10:10:2 format to RGBA8883. As shown in FIG. 14,the MSBs of each of the RGB channels are checked (block 1402) and if oneor more of these three MSBs are equal to one (‘Yes’ in block 1402) thena flag is set (block 1404), otherwise the flag is not set. This flag maybe referred to as the high-dynamic-range (HDR) flag because if at leastone MSB is equal to one then it is likely that the pixel data is HDRdata. HDR images can represent a greater range of luminance levels thannon-HDR images and HDR images are typically created by merging multiplelow- or standard-dynamic-range (LDR or SDR) photographs or by using aspecial image sensor. Hybrid log-gamma is an HDR standard that defines anon-linear transfer function in which the lower half of the signalvalues (this is the SDR part of the range) use an x² curve and the upperhalf of the signal values (this is the HDR part of the range) use alogarithmic curve and the reference white level is set to a signal valueof 0.5. Of the 10 bits of R/G/B data, the most significant bit indicateswhether a value is in the bottom half of the range (the SDR part) or inthe top half of the range (the HDR part).

As well as setting the flag or not, the pixel data is reduced from10-bits to 8-bits in different ways dependent upon whether one or moreof the MSBs for the RGB channels is one. If none of the three MSBs areequal to one (‘No’ in block 1402), then each of the 10-bit values forthe RGB channels is truncated by removing both the MSB (which is knownto be a zero) and the LSB (block 1410). If any of the three MSBs areequal to one (‘Yes’ in block 1402), then there are two different ways inwhich the 10-bit values may be reduced to 8-bits (in block 1406). In afirst example, the two LSBs may be removed from each 10-bit value and ina second example, the method as described above with reference to FIGS.12A-B may be used.

Where the method of FIG. 12A is used, n=10 and m=8 and the followingexample VHDL shows how the adjustment value is calculated (in block 1204and unit 1224):

function CorrectionsFor10to8(i : std_logic_vector(9 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00------11”) thenresults:= “01”; end if;  if std_match(i, “0-00----11”) then results:=results OR “01”; end if;  if std_match(i, “0-0-00--11”) then results:=results OR “01”; end if;  if std_match(i, “0-0-0-0011”) then results:=results OR “01”; end if;  if std_match(i, “1-1-1-1100”) then results:=results OR “11”; end if;  if std_match(i, “1-1-11--00”) then results:=results OR “11”; end if;  if std_match(i, “1-11----00”) then results:=results OR “11”; end if;  if std_match(i, “11------00”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor10to8;It is estimated that approximately 25 AND/OR gates are required toimplement the look-up logic unit 1224 for n=10 and m=8.

To reverse this mapping (e.g. as shown in FIG. 13A), such that n=8 andm=10, the following example VHDL shows how the adjustment value iscalculated (in block 1204 and unit 1224):

function CorrectionsFor8to10(i : std_logic_vector(7 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “0011----”) thenresults:= “01”; end if;  if std_match(i, “001-11--”) then results:=results OR “01”; end if;  if std_match(i, “001-1-11”) then results:=results OR “01”; end if;  if std_match(i, “110-0-00”) then results:=results OR “11”; end if;  if std_match(i, “110-00--”) then results:=results OR “11”; end if;  if std_match(i, “1100----”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor8to10;It is estimated that approximately 19 AND/OR gates are required toimplement the look-up logic unit 1224 for n=8 and m=10.

Irrespective of the values of the three MSBs for the RGB channels forthe pixel, the 2-bit alpha channel value is modified in the same way. Asshown in FIG. 14, the HDR flag is appended to the existing 2-bit value(block 1408), making the output alpha channel value 3-bits.

The method of FIG. 14 may be implemented on a per-pixel basis, althoughin variations on this method, the decision (in block 1402) that leads tothe setting of the HDR block may alternatively be performed lessfrequently, e.g. on a sub-block or mini-block basis.

As noted above, where the method of FIG. 14 is used as a pre-processingstep, the methods implemented in the architectures shown in FIGS. 2A-Cmay be modified slightly. In particular, the compression threshold usedby the test and selection unit 206 (in FIG. 2A) or the test and decisionunit 210 (in FIGS. 2B-C) is calculated based on the size of the originaldata, prior to pre-processing, or alternatively, the threshold may bemodified to account for the reduced size of the alpha channel values inthe source data (i.e. 3-bits rather than 8-bits). This means, forexample, that instead of defining the compression threshold in terms ofa ratio, the threshold that is used may be defined in terms of the sizeof the compressed data block (e.g. 128 bytes) rather than a ratio.Additionally, where the reserve compression unit 204 implements themethod described above with reference to FIGS. 3A and 4-11, this lossycompression method is also modified, as described below, as aconsequence of the alpha value only comprising 3-bits, one of which isthe HDR flag.

In a first variation on the lossy compression method of FIG. 3A, theremay be three constant alpha modes that are used (in block 306) alongwith the variable alpha mode (in block 308). This is because there areonly four possible alpha values, A, (instead of the 256 values wherealpha is an 8-bit value) and hence the mode can be determined in a lesscomplex manner than described above with reference to FIGS. 5A-B. Eachof the constant alpha modes has a different pre-defined value ofconstalphaval which can be specified by using a different field valuethat is inserted into the block mode bits 408 (in FIG. 4C) and hence thevalue of constalphaval does not need to be stored separately (e.g. infields 670-673 as is the case in the earlier description). As notedbelow, where the alpha value is zero, this uses the same field value asvariable alpha; however as such a block is fully transparent, the RGBmay be less important than for other alpha values and in this mode, thedata is encoded as if alpha were variable per pixel.

Field value Interpretation 0b00 Constant alpha Fully opaque: A = 0 × 30b01 Constant alpha Transparent: A = 0 × 2 0b10 Constant alphaTransparent: A = 0 × 1 0b11 Variable alpha or Constant alpha, fullytransparent: A = 0 × 0

The constant alpha modes (of block 306) are then implemented using themethod of FIG. 11, i.e. using the encoding patterns and three palettecolours; however, in this case the palette colours are each representedas RGB676 plus a 1-bit HDR flag and hence block 1112 involves adifferent conversion to that described above. To perform thisconversion, for the G channel, the value is truncated and for the R andB channels, the method described above with reference to FIGS. 12A-B mayagain be used, with n=8 and m=6, or alternatively the values may simplybe truncated.

The adjustment value for n=8 and m=6 may be determined (in block 1204and look-up logic unit 1224) according to the following example VHDL (orits functional equivalent):

function CorrectionsFor8to6(i : std_logic_vector(7 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “00----11”) thenresults:= “01”; end if;  if std_match(i, “0-00--11”) then results:=results OR “01”; end if;  if std_match(i, “0-0-0011”) then results:=results OR “01”; end if;  if std_match(i, “1-1-1100”) then results:=results OR “11”; end if;  if std_match(i, “1-11--00”) then results:=results OR “11”; end if;  if std_match(i, “11----00”) then results:=results OR “11”; end if;  return results; end functionCorrectionsFor8to6;It is estimated that approximately 19 AND/OR gates are required toimplement the look-up logic unit 1224 for n=8 and m=6.

The reverse mapping (e.g. as shown in FIG. 13A), such that n=6 and m=8,may be implemented according to the following example VHDL (or itsfunctional equivalent):

function CorrectionsFor6to8(i : std_logic_vector(5 downto 0))  returnstd_logic_vector is  variable results:std_logic_vector(1 downto 0);begin  results:=(others=>‘0’);  if std_match(i, “0011--”) then results:=“01”; end if;  if std_match(i, “001-11”) then results:= results OR “01”;end if;  if std_match(i, “110-00”) then results:= results OR “11”; endif;  if std_match(i, “1100--”) then results:= results OR “11”; end if; return results; end function CorrectionsFor6to8;

Furthermore, the four colour mode (in FIG. 11) is modified such that thepixels are represented as RGB554 (instead of RGB555 as in block 602,described above) along with a 1-bit HDR flag. The reduction of bitvalues from 8-bits to 5-bits or 4-bits may be performed as describedabove (e.g. with reference to FIGS. 12A-B).

Similar modifications in terms of data format are also made to thevariable alpha mode, as described above with reference to FIG. 9. Inparticular the three palette entries are encoded with a 1-bit HDR flag,2 alpha bits and RGB665 (in blocks 910-912) and the four colour modeuses a 1-bit HDR flag, 2 alpha bits and RGB444 (in block 916).

As a consequence of the different data formats used, the data is packedinto the data fields (i.e. the 60-bit data fields 660-663 as shown inFIG. 6C) in a slightly different way to that described above and thiscan be described with reference to FIGS. 15A-D. FIGS. 15A and 15B relateto the constant alpha mode (as packed in block 1114 of FIG. 11) andFIGS. 15C and 15D relate to the variable alpha mode (as packed in block914 of FIG. 9). FIGS. 15A and 15C show the packing where the four colourmode (of blocks 916 and 602) are used and FIGS. 15B and 15B show thepacking where the encoding patterns (of blocks 908-912 and 1112) areused. The packing arrangement shown in FIG. 15C replaces that shown inFIG. 100 and described above. The packing arrangement shown in FIG. 15Dreplaces that shown in FIG. 10B and described above.

FIG. 16 shows a flow diagram of a second example method of convertingpixel data from 10-bit (e.g. RGBA10:10:10:2) format to 8-bit format(e.g. 8888 format). This method may, for example, be used as apre-processing step for RGBA10:10:10:2 format data before it is input tothe primary compression unit 202. The method of FIG. 16 can be describedwith reference to the schematic diagram in FIG. 17. As shown in FIG. 17,the input pixel data comprises four input channels 1702-1708, which arereferred to here as RGBA channels although this data may alternativelyrepresent other channels. The output from the method are four 8-bitpseudo-channels.

The RGB input channels 1702-1706 each comprise 10 bits and the A (alpha)input channel 1708 comprises two bits. The RGB channels are eachtruncated by removing one or more LSBs (block 1602) and in the methodshown, the R and B channels are truncated to 7 bits 1710, 1714 byremoving three bits from each channel, 1711, 1715 and the G channel istruncated to 8 bits 1712 by removing two bits 1713. The 8-bit channel1712 formed by truncation of the G channel forms one of four outputpseudo-channels (block 1604).

The alpha channel data is optionally Gray coded (block 1606) before onebit 1716 is appended as a new MSB on the truncated R data 1710 to formanother one of the four output pseudo-channels 1720 and the other bit1718 from the (optionally Gray coded) alpha channel data is appended asa new MSB on the truncated B data 1714 to form a further one of the fouroutput pseudo-channels 1722 (block 1608).

The fourth of the four output pseudo-channels 1730 is formed from errorvalues. In one example, to generate the error values, the truncatedRGB787 data 1710, 1712, 1714 is replicated so that each channelcomprises 10 bits 1724, 1726, 1728 (block 1610). As shown in FIG. 17,this bit replication involves copying the three (for R and B) or two(for G) MSBs and appending them as new LSBs to the truncated data. Thesenew LSBs of the updated 10:10:10 data are then combined with acorresponding number of the original LSBs of the input data channels1711, 1713, 1715 to generate the error values (block 1612). One way tocombine these three or two bit values would be using a subtractionoperation (modulo 8 for R and B and modulo 4 for G), as follows:Red error=(3 new red LSBs−3 LSBs of input red data)mod 8Green error=(2 new green LSBs−2 LSBs of input green data)mod 4Blue error=(3 new blue LSBs−3 LSBs of input blue data)mod 8However, use of an XOR, as shown in FIG. 17, provides a more compacthardware implementation. In other examples, any reversible function maybe used. In an alternative example, instead of performing the bitreplication of the MSBs as new LSBs to the truncated data, the three(for R and B) and two (for G) MSBs of the truncated RGB787 data may bedirectly combined with the original LSBs of the input data channels1711, 1713, 1715 to generate the error values in block 1612. As above,the combination may be through a subtraction, XOR, or any suitablereversible function.

Having generated the three error values (in block 1612), these arepacked together to form the fourth pseudo-channel 1730 (block 1614).This may be formed by concatenating the three error values, oralternatively, the bits from the error values may be interleaved suchthat the three LSBs 1732 of the pseudo-channel 1730 comprise the LSBfrom each of the three error values, the two MSBs 1734 of thepseudo-channel 1730 comprise the MSB from each of the red and blue errorvalues and the middle three bits 1736 comprise the remaining three bits,one from each of the three error values, as shown in FIG. 17. By usingthis arrangement of bits in the fourth pseudo-channel 1730, thereduction in image quality that results from the discarding of one ormore of these bits (e.g. as described below with reference to FIG. 18A)is minimised. For example, if the 3 LSBs of the fourth pseudo-channel1730 are discarded, this results in the loss of a single MSB of theerror value for each of the RGB channels, rather than the loss of theentire error value for one of the channels (e.g. the B channel).

The following pseudo-code (or its functional equivalent) may be used toimplement the method of FIG. 16:

r′[9:0]:=Replicate(Truncate(r[9:0], 10,7),7,10);

g′[9:0]:=Replicate(Truncate(g[9:0], 10,8),8,10);

b′[9:0]:=Replicate(Truncate(b[9:0], 10,7),7,10);

r_x[2:0]:=r′[2:0] xor r[2:0];

g_x[1:0]:=g′[1:0] xor g[1:0];

b_x[2:0]:=b′[2:0] xor b[2:0];

a′[1:0]:={a[1], a[1] xor a[0]}

ch0_Red:={a′[0], r′[9:3]}

ch1_Grn:=g′[9:2]

ch2_Blu:={a′[1], b′[9:3]}

ch3_Lsb:={r_x[2], b_x[2], r_x[1], g_x[1], b_x[1], r_x[0], g_x[0],b_x[0]}

Whilst FIGS. 16 and 17 and the pseudo-code above show the bits from thealpha channels being appended as MSBs on the truncated R and B channeldata (in block 1608), in other examples the alpha bit may be inserted atdifferent positions (e.g. as an LSB or in the middle of the truncated7-bit data). However, the data compression that can be achieved, whenthis method is used in combination with a lossless compressiontechnique, may be improved by positioning the alpha bit as the MSB.

The four 8-bit pseudo-channels 1720, 1712, 1722, 1730 generated usingthe method of FIG. 16 can then be fed into a data compression methodthat is designed to operate on RGBX8888 data, such as the losslesscompression method implemented by the primary compression unit 202 inFIG. 2.

As described above, use of Gray coding of the alpha channel (block 1606)is optional. By using Gray coding (in which only a single bit changes asthe value increments by one), the amount of data compression that can beachieved using a lossless compression method (such as described belowwith reference to FIGS. 20A-B, 21A-B and 22 and/or in UK Patent NumberGB2530312) is increased (e.g. by 1-2%). This is because it reduces theinfluence the alpha channel, which is often uncorrelated with the R, Gor B data, has on both colour and/or spatial decorrelation steps, whichare common first (or early) steps in lossless compression. Colourchannel decorrelation often uses colour transforms such as:R*=R−G mod 2⁸G*=GB*=B−G mod 2⁸A*=AWhere the pre-processing of FIG. 16 has been performed, thepseudo-channels do not correspond to the original colour data andinstead the alpha data is combined into the red and blue channels. Wherethe value of alpha changes, the values of R* and B* can changesignificantly from one pixel to the next, making the data harder tocompress; however, through the use of Gray coding, any single valuechange affects only R* or B* and not both.

As was the case for the method of FIG. 14, if the method of FIG. 16 isused as a pre-processing step, the methods implemented in thearchitectures shown in FIGS. 2A-C may be modified slightly. Inparticular, where the method of FIG. 16 is used as a pre-processing stepfor the primary compression unit 202, an additional option may beconsidered by the test and selection unit 206 in FIG. 2A or the test anddecision unit 210 in FIG. 2B or 2C. In the methods described above,these test and decision/selection units 206, 210 compare the compresseddata generated by the primary compression unit 202 to a compressionthreshold and if the compression threshold is satisfied, the compresseddata generated by the primary compression unit 202 is output. If thethreshold is not satisfied, data compressed by the reserve compressionunit 204 (or sub-units 204A, 204B) is output instead. Where thepre-processing method of FIG. 16 is used, if the compression thresholdis not satisfied by the compressed data generated by the primarycompression unit 202, the compressed data may be further reduced in sizeby discarding all of the data corresponding to the fourth pseudo-channel1730 (i.e. the pseudo-channel formed from the error values). In such animplementation, the compressed data generated by the reserve compressionunit 204 (or sub-units 204A, 204B) is only output if, having discardedthe entirety of the data corresponding to the fourth pseudo-channel, thecompression threshold is still not satisfied. This reduces the amount ofdata that is lost in data compression (since the lossy compressionmethod in the reserve compression unit 204 is used less often) and henceimproves the quality of the subsequently decompressed data.

Instead of discarding the entirety of the fourth pseudo-channel, the bitprediction technique described below with reference to FIG. 19A, mayinstead be used to partially discard bits prior to compression. In afurther variation, a subset of the bits of the fourth pseudo-channel maybe discarded and then the data may be fed back into the primarycompression unit 202.

FIG. 18A shows a flow diagram of a data compression method whichcombines the pre-processing method of FIG. 16 (block 1802) with alossless data compression method (block 1804), such as described inGB2530312 and/or below with reference to FIGS. 20A-B, 21A-B and 22. Asnoted above, some or all of the fourth pseudo-channel may be discarded(in block 1804) to increase the amount of data compression. Thisdiscarding may be implemented, as described above, to ensure that acompression threshold is satisfied (as in FIGS. 2A-C) or it may beimplemented to improve compression but without any defined threshold ortest and decision process. In order that the decompression hardware canoperate in the same way irrespective of whether some or all of thefourth pseudo-channel data is discarded (in block 1804), when discardingthat data, one or more zeros (e.g. a single byte of data comprising allzeros) or another constant, pre-defined, sequence of bits, may beretained for that channel. Alternatively, a special packing code orother means may be used to indicate to the decompressor that the fourthpseudo-channel data has been discarded. The decompression can thenproceed as described below with reference to FIG. 18B.

As shown in FIG. 18B, the compressed data is initially decompressed(block 1810) using the inverse of the lossless compression method usedwhen compressing the data (in block 1802). This operation outputs datain 888X format, as the fourth channel may be 8-bits or fewer, dependentupon how much data was discarded (as described above) and may be totallyabsent. The fourth channel is then reconstructed such that it comprises8-bits for the pixel (block 1812) and this may be implemented by addingzeros or another pre-defined sequence of bits. As described above, oneor more zeros or the sequence of bits may be present in the compresseddata and used in the reconstruction (in block 1812) or alternatively,the reconstruction may proceed based on a known convention without anyneed for any bits relating to the fourth channel in the compressed data.

The alpha channel data can then be extracted (block 1814) by removingthe MSBs from the first and third channels and concatenating them(followed by optional Gray decoding, where this was used in the datacompression), leaving 787 format data which is expanded to 10:10:10format by bit replication (block 1816). The error values from the fourthpseudo-channel are then applied using an inverse of the function used togenerate those error values (in block 1612). In the example shown inFIG. 18B, the LSBs from each channel (e.g. 3 bits from the first andthird channels and two bits from the second channel) are combined withthe corresponding bits from the fourth channel using an XOR logic gateor equivalent logic block (block 1818). Where any bits from the fourthchannel were previously discarded, XORing with a zero will not changethe value of the LSB. In examples where a pre-defined sequence of bitsis used in the reconstruction that does not comprise all zeros, analternative function may be used both to generate the error values whencompressing the data (in block 1612) and to apply the error values whendecompressing the data (in block 1818).

In an example implementation, both the pre-processing methods of FIGS.14 and 16 may be used. In such an example, the pre-processing methodshown in FIG. 14 and described above may be used to pre-processRGBA10:10:10:2 format data before it is input to the reserve compressionunit 204 or may be implemented prior to or by the initial reservecompression sub-unit 204A. The pre-processing method shown in FIG. 16and described above may be used to pre-process RGBA10:10:10:2 formatdata before it is input to the primary compression unit 202.

The methods described above are described with reference to RGBA or RGBXdata but as noted above, the methods may also be used with data otherthan image data, where three of the four channels (e.g. the RGB channelsabove) are correlated across channels in some way (e.g. colour orspatially correlated). For some types of data, such as YUV or YCbCr data(where YCbCr data is digital luma and two chroma components and YUV isits analogue counterpart, although it may also be used to mean YCbCr)which are frequently arranged in two or three planar formats (e.g.dependent upon whether the U and V data is separate or interleaved),there may be little or no correlation across channels within a datablock, as the channels are often assembled by gathering disjoint, albeitadjacent, single- or dual-channel pixel blocks e.g. multiple tiles ofonly Y, only U or only V or only UV are packed as the channels of theblock. As a consequence, whilst the compression methods described abovecan be used, and despite that the palette modes of the lossy compressionmethod (e.g. as described above with reference to blocks 908-912 of FIG.9) work well where the data has strong spatial correlation as well asgood correlation between channels, they do not work well for YUV orYCbCr data (or other uncorrelated data types) as it is difficult topredict a value from its neighbours and therefore it relies too heavilyon the four colour mode (block 916 of FIG. 9).

FIG. 19A is a schematic diagram of a data compression unit whichincreases the amount of compression that is achieved compared to aprimary compression unit 202 that implements only a lossless compressionmethod such as the method described in UK Patent Number GB2530312 or themethod described below with reference to FIGS. 20A-B, 21A-B and 22. Thisdata compression unit shown in FIG. 19A may be used as the primarycompression unit 202 in the architectures of any of FIGS. 2A-C,particularly where the input data is not correlated across channels andtherefore cannot greatly benefit from colour decorrelation or colourpalettes. Furthermore, as the data compression unit of FIG. 19A mayoperate separately on each channel, it may be implemented within theprimary compression unit 202 for only a subset of the channels (e.g. forthe alpha channel or the fourth pseudo-channel created using the methodof FIG. 16) whilst the remaining channels are compressed using alossless compression method such as the method described in UK PatentNumber GB2530312 or the method described below with reference to FIGS.20A-B, 21A-B and 22. In examples where the data compression unit of FIG.19 is used as the primary compression unit 202 in the architectures ofany of FIGS. 2A-C, there is less reliance upon the reserve compressionunit 204 which may involve more severe quantisation and hence result ina larger reduction in image quality.

The data compression unit shown in FIG. 19A may alternatively be usedseparately from the architectures shown in FIGS. 2A-C, i.e. without anytest and selection/decision unit 206, 210 or reserve compression unit206 and where there is no need to guarantee that a compression thresholdis met. Instead, by using the data compression unit shown in FIG. 19A,the amount of data compression achieved overall may be increased and aparticular compression ratio may be targeted. The data compression unitshown in FIG. 19A may also be used with compression methods other thanthose described in UK Patent Number GB2530312 or the method describedbelow with reference to FIGS. 20A-B, 21A-B and 22 and/or for compressionof data other than image data (e.g. for any data that has a compressionalgorithm as well as multiple levels of quantisation, where furtherquantisation improves the efficiency of the compression algorithm).

As shown in FIG. 19A, source data block (which for the purposes of thefollowing description is assumed to be in 8888 format) is split up andeach channel may be considered separately (as indicated by the dottedbox in FIG. 19A). Alternatively, two or more channels (or any subset ofthe channels) may be considered together (and in this case, the totalsize that is determined relates to all channels being consideredtogether). For example, RGB data may be considered together, with asingle level of quantisation (i.e. a single mapping parameter) beingchosen uniformly across the three colour channels. The followingdescription refers to consideration of each channel separately by way ofexample only.

The data for a single channel is input to both a bit predictor element1902 and an x-bit remapping element 1904. The operation of the x-bitremapping element is controlled based on an input from the bit predictorelement 1902, referred to as the mapping parameter. This mappingparameter specifies a data conversion or truncation operation that isperformed by the x-bit remapping element 1904 and in particular, thismapping parameter specifies the value x. By selecting a lower value ofx, the amount of data compression that is achieved is increased,however, the accuracy is reduced since the remapping is a lossyoperation. The data conversion operation that is performed by the x-bitremapping element 1904 may also include operations such as low passfiltering where the mapping parameter, x, determines how heavilyfiltered the data is.

The operation of the bit predictor element 1902 is shown in FIG. 19B.The bit predictor element 1902 takes the data for the channel (e.g.8-bits per pixel for each pixel in the data block), compresses it usingthe same data compression method used in the data compression element1906 (block 1922), however, some operations of the data compressionmethod may be omitted (e.g. those operations after entropy encoding).For example, the bit predictor element 1902 may not pack the compresseddata into a compressed data block but may instead just determine thetotal size of the compressed data (block 1924). This total size is thencompared to a plurality of thresholds (e.g. A_(Y), B_(Y), C_(Y), D_(Y))which may, for example, be stored in a look-up table, and based on thecomparisons, a mapping parameter (i.e. a level of quantisation and hencea value of x) is determined (block 1926) and output.

In examples where the data compression within the bit predictor element1902 is performed using a lossless compression method such as the methoddescribed in UK Patent Number GB2530312 or the method described belowwith reference to FIGS. 20A-B, 21A-B and 22, the bit predictor element1902 may compress the data (in block 1922) using wavelet transformation,sign remapping and group length generation; however, group length codes,packing mode codes and the actual packing of data may be omitted as theyare not needed in order to generate the size of the compressed block. Insuch examples, the total size of the compressed data for the channel maybe given by the size of the variable length data of the channel (see thedescription of FIGS. 20A-B, 21A-B and 22 below). This size may becalculated by summing four times the coefficient length for each group(which comprises a 2×2 set of values) other than the top-left. The groupsizes may be determined by combining neighbouring values into groups offour and then the number of bits required to encode the largest of theseis used as the group bit-depth and is encoded in the block. The size ofthe group is then four times this bit-depth. The top left group istreated differently as the reference value is always stored at fullprecision and its size may be determined from a look-up table. In otherexamples, any suitable data compression method may be used within thebit predictor element 1902 and the data compression element 1906.

The comparison to the plurality of thresholds (in block 1926) may, forexample, be implemented as set out in the following pseudo-code (or itsfunctional equivalent), where Chan Size is the total size of thecompressed data for the channel:

If (Chan Size ≤ A_(Y)) x = 8 Else if (Chan Size ≤ B_(Y)) x = 7 Else if(Chan Size ≤ C_(Y)) x = 6 Else if (Chan Size ≤ D_(Y)) x = 5 Else x = 4and in various examples, a look-up table may be used. The pseudo-code(or look-up table) comprises at least two rows but may comprise manymore rows. Furthermore, whilst the value of x is described herein asreferring to an actual bit depth, in other examples the mappingparameter, x, may correspond to a set of different quantisations.

The values of each of the plurality of thresholds (e.g. A_(Y), B_(Y),C_(Y), D_(Y)) used by the bit predictor element 1902 may be determinedby compressing a large training set of images multiple times usingdifferent configurations of thresholds and analysing the resultant sizesof the compressed data, or the score of an image quality metric for agiven target compression ratio. The thresholds may be set to achieve aparticular compression threshold or target compression ratio or may becalibrated to minimise certain artefacts of loss compression such asblock artefacts or banding. The thresholds may also be set such thatonly those blocks that do not compress well are quantised.

Having determined the mapping parameter (i.e. value of x) in the bitpredictor element 1902, the x-bit remapping element 1904 converts theinput data from its original format, which may be 8-bits per pixel tox-bits per pixel, where x is the mapping parameter. This conversion may,for example, use truncation or may alternatively use the methoddescribed above with reference to FIGS. 12A-B. As described above, inother examples, the mapping parameter may correspond to a set ofdifferent quantisations rather than an actual bit depth and in suchexamples, the remapping element 1904 converts the input data based onthe mapping parameter. In other examples, the bit depth might not bealtered but instead, the information content may be reduced by low passfiltering, under the control of the mapping parameter, x. Alternatively,a combination of both filtering and bit precision reduction may beemployed.

The data that is generated by the x-bit remapping element 1904 is inputto the data compression element 1906 that then compresses the data usinga data compression method, such as the lossless compression methoddescribed in UK Patent Number GB2530312 or the method described belowwith reference to FIGS. 20A-B, 21A-B and 22. It will be appreciated thatwhilst this method may be described as lossless and may be losslessbased on the data input to the data compression element 1906, theresultant compressed data is not lossless with reference to the sourcedata, since the conversion performed by the x-bit mapping element 1904may be lossy.

In order that the data can subsequently be decompressed, it is necessaryto include the value of x the compressed data in some way, so that thecorrect inverse mapping may be used. FIG. 19C shows an example of theway in which data for the four channels may be packed into a compresseddata block (in block 1910). As shown in FIG. 19C, the size of the datafor each channel 1930-1933 may be different where the data compressionmethod includes entropy encoding. The 8-bit mode field 1940 comprises a2-bit packing mode for each channel. One of the possible packing modesmay be used to indicate that x<8; however, given the available number ofbits, it may not be possible to include the value of x for the channelin the packing mode without increasing the size of the mode field 1940.Consequently, the bit depth encoding element 1908 may encode the valueof x (where it is less than 8) in the data generated by the datacompression element 1906. For example, the value of x may be encodedwithin the reference value field of the compressed channel, e.g. bychanging the least significant unused bit from 0 to 1, i.e. a unaryencoding. As described below with reference to FIGS. 20A-B, 21A-B and22, the reference value is a data value which is not entropy encoded butis included in a specified reference value field of the encoded data (asgenerated by the data compression element 1906). This reference valuemay, for example, be modified to encode the value of x as follows (where‘0b’ is a binary value indicator and e,f,g,h,i,j,k are used to identifybits that are used to encode the reference value):

x Original value Encoding 7 0b0efghijk 0b1efghijk 6 0b00fghijk0b01fghijk 5 0b000ghijk 0b001ghijk 4 0b0000hijk 0b0001hijk

If the compressed data has no redundancy that can be exploited, then thevalue x may instead be packed alongside the compressed data, e.g.occupying a header byte. This can be read at the point of decompressionto determine which inverse mapping to perform after decompressing theblock in order to map the data back up to its original precision.

FIG. 19A also shows an optional y-bit remapping element 1912. This maybe used to reduce the number of bits per pixel in the source data forthe channels that are used by the bit predictor element 1902. The y-bitremapping element 1912 may use truncation or the method described abovewith reference to FIGS. 12A-B to reduce the bit depth of the sourcedata. The bit depth, i.e. the value of y, may be a value between 4 and8, where y=8 provides the most accurate predictions (i.e. in terms ofselecting a value of x to satisfy an overall compression threshold ortarget) but is most expensive in terms of operations performed by thebit predictor element 1912. A value of y=4 provides the least accuratepredictions but is least expensive in terms of hardware and operationsimplemented by the bit predictor element 1902. Similar to the value x,the value y may correspond to some other kind of quantisation, e.g. alow pass filter. By removing noise from the data the accuracy of thepredictor's result can be improved, and can be less susceptible tochange due to small perturbations in the original data.

It will be appreciated that whilst the description of FIGS. 19A-C aboverefer to source data comprising 8-bits per pixel per channel, themethods may also be used where there are more or fewer bits per pixelper channel, e.g. 10, and this may result in more or fewer thresholdsbeing used (in block 1926).

In examples where the source data is 10-bit data, rather than 8888 data,the data compression unit may include an additional pre-processingelement, not shown in FIG. 19A. This pre-processing element may performa data conversion similar to that described above with reference toFIGS. 16 and 17. The 10-bit values are truncated and then expanded backto 10-bit values using bit replication, thereby generating new LSBs anderror values are generated by XORing the LSBs of each original 10-bitvalue with the new LSBs of the corresponding truncated expanded value.The 8 MSBs of three tiles (which correspond to three input channels)form the first, second and third pseudo-channels whilst the error valuesare packed into the fourth pseudo-channel as described above and shownin FIG. 17. One set of thresholds is then used in the bit predictorelement 1902 for the first three pseudo-channels and these thresholdsmay be different from those used for 8888 data. A second set ofthresholds (which also may be different from those used for 8888 data)is used for the fourth pseudo-channel and the quantisation options maybe reduced, e.g. to 6 and 3 bits only. Where the data compression unitof FIG. 19A is used as the primary data compression unit 202 in any ofthe architectures of FIGS. 2A-C, the test and selection/decision unitsalso operate differently. It is initially checked whether all fourpseudo-channels fit within the target size, in which case they areincluded. If they do not all fit, it is checked whether the first threepseudo-channels fit with a single additional byte, and if so the fourthpseudo-channel is replaced by a constant zero channel (occupying asingle byte). If this test still does not satisfy the target compressionthreshold, then the output from the reserve compression unit is used.The data can subsequently be decompressed using a modified version ofthe method described above with reference to FIG. 18B.

In examples where the data compression unit shown in FIG. 19A targets aparticular compression rate (e.g. 50%) and uses the lossless compressionmethod described in UK Patent Number GB2530312 or the method describedbelow with reference to FIGS. 20A-B, 21A-B and 22, the RAW mode of thecompression method may be modified to store data at a differentprecision, e.g. 4 bits rather than 8 bits.

The data compression unit shown in FIG. 19A is a versatile unit that isable to compress multiple types of data, e.g. YUV, RGB(A), non-imagedata, etc. It is applicable to various combinations ofquantisation/lossy compression and can be used to improve compressionrates or target a specific compression ratio. The thresholds that areused may be calibrated to produce desired properties, such as minimisingblock artefacts or banding or to optimise a given metric score, and thethresholds may be set conservatively, so as to minimise use of a reservecompression unit that performs lossy compression (where provided). Thedata compression unit is also very configurable and the cost/complexityof the bit predictor element 1902 can be traded off against accuracy andperformance of the data compression. Additionally it is very scalableand may, for example, be applied only to subsets of the data, e.g. thealpha channel only.

A first example of a lossless compression method which may beimplemented in the primary compression unit 202 (shown in FIGS. 2A-C anddescribed above) and within the data compression unit shown in FIG. 19A(e.g. within both the bit predictor element 1902 and data compressionelement 1906 is described in UK patent number 2530312. UK patent number2530312 describes a data compression system in which 8×8 blocks of imagedata are compressed by performing colour decorrelation, spatialdecorrelation and entropy encoding, and then packing the resultingencoded data values into data packets for storage. In the systemdescribed in that patent, a row of data values from a block is processedper iteration. A modified version of Exponential Golomb coding is usedfor the entropy encoding wherein each coefficient stored a prefix toindicate a range of values that were then identified by a suffix. Havinga prefix and suffix for each encoded coefficient makes decoding multiplevalues simultaneously difficult because it is not simple to quicklydetermine the positions of the boundaries between encoded bits.

A second example of a lossless compression method which may beimplemented in the primary compression unit 202 (shown in FIGS. 2A-C anddescribed above) can be described with reference to FIGS. 20A-B, 21A-Band 22. As described in UK patent number 2530312 and shown in FIGS.20A-B, data compression may comprise the following steps (notnecessarily in this order): division into blocks, format conversion(block 2024), colour channel decorrelation (block 2024, e.g. asdescribed above), spatial decorrelation (block 2026), entropy encoding(block 2028) and then storage.

The entropy encoding (in block 2028) is performed according to avariable-length coding (VLC) scheme, such that the entropy encodedvalues will most likely not all have the same number of bits. Ingeneral, more probable values are encoded with fewer bits. In this way,it is likely that the total number of bits used to encode all of thedata values in a data block will be reduced by the entropy encoding.There are some situations where spatial decorrelation can expand data,and these situations can be treated as special cases (i.e. the data istreated differently to other cases) in order to limit the expansion.

VLC encoding can be reasonably straightforward to perform at a highrate, but VLC decoding at a matching rate can be significantly moredifficult because the bit-boundaries between contiguously stored encodeddata values are not known until the encoded data is analysed, i.e. thelength of the encoded data values is not known until the encoded data isanalysed. In particular, when encoding, multiple symbols can be mappedindependently to their corresponding codes and code lengths, and thenmerged into a contiguous output bit stream. However, typically whendecoding, each code in the encoded bit stream is examined sequentiallyin order to determine its length which, in turn, determines the locationof the start of the next code. In other words, the bit-boundariesbetween the different encoded data values need to be found.

In this second example method, entropy encoding is performed on sets ofdata values (e.g. 2×2 sets of data values). For each set of data valuesan indication is included in an encoded output (e.g. in a header sectionthereof) to indicate how many bits are used for each of the encoded datavalues representing the set of data values. The encoded data values arethen included in the encoded output in accordance with the indicatednumbers of bits. This system means that the decoding of the encoded datavalues is simplified (compared to the system of UK patent number2530312) because a simple read of the indications (e.g. in the header)allows the bit boundaries between different encoded data values to bedetermined. In other words the indications allow the sizes of thecorresponding encoded data sections to be quickly determined (e.g. injust a single clock cycle). This is described below in more detail withreference to FIGS. 21A-B and 22.

FIG. 21A shows an 8×8 block of data values 2100 which have beenspatially decorrelated and remapped by one of the spatial decorrelatingmodules 2004. The data values relate to respective pixel values. Amethod of performing entropy encoding on the block of data values at anentropy encoding module 2006 is described with reference to the flowchart shown in FIG. 22. The entropy encoding module 2006 outputs anencoded data output 2110 (as shown in FIG. 21B) which comprises encodeddata values to represent the data values in the 8×8 block 2100. Theencoded data output 2110 has a variable length depending on thecompression that can be achieved by the entropy encoding process. Inparticular, in an example in which each received data value in the block2100 comprises 8 bits (i.e. 1 byte), the encoded data output 2110comprises a reference value field 2112 which is 8 bits (1 byte) wide, asize indication field 2114 which is 48 bits (6 bytes) wide, and avariable size field which, in this example has been configured tomaintain 8-bit alignment of fields, is anywhere between 8 bits (1 byte)and 448 bits (56 bytes) wide. If the entropy encoding process is notable to losslessly represent the data values in the data block 2100without exceeding the number of bits available in the encoded dataoutput 2110 (i.e. 504 bits or 63 bytes) then the data values in the datablock 2100 are stored in raw form in the encoded data output, with 8bits per data value. A header section of a data packet in which theencoded data output is placed has an indication to indicate the formatof the encoded data within the data packet. For example, the indicationmay comprise two bits such that up to four different encoding modes canbe set, which may include a raw data format, an entropy encoded formatand a constant format (in which all of the data values within a blockhave the same value, such that the value can be stored once (using 8bits) to represent the value of each of the data values within theblock.

The block of data values is received at an entropy encoding module 2006(block 2202). The rows and columns of the block are arranged as shown inFIG. 21A. The block of data values 2100 comprises a plurality of sets ofdata values (most of which in this example are 2×2 sets of data values(relating to 2×2 sets of pixel values), but in other examples, the setscould be different shapes and/or sizes). The block 2100 comprisesfifteen 2×2 sets of data values, and one particular set (in the top leftof the block) which comprises just three data values. The top left datavalue (AO) is not entropy encoded, and instead the data value atposition AO is included in the reference value field 2112 of the encodeddata output 2110 (block 2204). Since the top left set represents asimilar sized set compared to the other sets in the block, but minus thedata value which is to be used as the reference value, the top left setincludes fewer (e.g. 3 rather than 4) data values than each of the othersets.

In the example shown in FIG. 21A, the rows in the block have the orderA″, E″, C″, G″, B″, D″, F″, H″, and the columns in the block have theorder 0, 4, 2, 6, 1, 3, 5, 7 for the purposes of the entropy encoding.Other examples, may order the rows and columns differently to that shownin FIG. 21A. The example order shown in FIG. 21A does not exactly matchthe order in which the rows are processed by the spatial decorrelationmodules (e.g. row 6 is shown above rows 1 and 3). With the exception ofthe top left 2×2 set (which only groups three values and is encodedslightly differently), each aligned 2×2 set of values will be encodedtogether. This example grouping has been chosen such that: (a) thevalues within a set belong (where possible) to the same spatialfrequency, and (b) the values within a set come from pixels that are,where possible, nearby spatially. This is done because these are morelikely to be “similar” in magnitude and, hence, the sets can becompressed more effectively.

As described above, the data values are unsigned with a distributionwhich is biased towards zero (due to the colour correlation and spatialdecorrelation processes). Therefore, data values are likely to haveleading zeroes. Therefore, the data values can be compressed simply byremoving one or more leading zeroes from the data values (wherepossible). An indication is used to indicate how many leading zeroeshave been removed.

The top left set of four data values are treated differently in that:(a) the top left pixel is used as the reference and stored separately,and (b) the remaining three values have a different encoding scheme thathas been “trained” on a large set of image data so that: (i) foralignment purposes, the total number of bits used to represent the threevalues is a multiple of 4 (this matches the 2×2 blocks), and (ii) thetotal storage cost (for the training image set) has been minimised. Theparticular set of data values in the top left of the block is processed(block 2206). Specifically, an indication for the particular set isincluded in the size indication field 2114 (block 2208). Each indicationin the size indication field has three bits, for a respective set ofdata values in the block 2100. This 3-bit indication can be used as anindex into a Look Up Table to retrieve a number of bits for each datavalue in the particular set, such that the number of bits for each valueis sufficient to store that value. There are sixteen sets of data valuesin the block 2100, such that the size indication field comprises 48 bits(i.e. 3*16 bits), or 6 bytes. The fifteen sets of four data values in ablock will be encoded with numbers of bits which are multiples of four(because, as described below, each encoded data value in a set of fourdata values has the same number of bits), and on any given row each ofthese fifteen sets contribute a multiple of two bits. To maintain aconvenient alignment of compressed data (as restricting alignment ofdata can reduce hardware complexity) restrictions are applied to theavailable choices for the top-left set which only has 3 elements. Thetop left set has three data values, one of which (E0) is in row 0. Forhardware efficiency, it is desirable that the total data for a row is aneven number of bits, so the possible lengths of the encoded data valueswhich can be used to represent this data value are restricted to evennumbers. Similarly, the combined lengths of the two encoded data valueswhich are used to represent the other two data values of this set (A4and E4) sum to an even value. Furthermore, in this example, the combinedlength of all the encoded data values in the encoded data output will berounded up to the nearest byte (so that the encoded data outputs arealigned on byte boundaries), and all of the other sets of data values inthe block. Therefore, the total number of bits used to encode the topleft set of data values is also a multiple of four.

With a size indicator including three bits, eight sets of lengths forthe encoded data values can be set. For example, the table below showssome possible lengths of the encoded data values which may berepresented by the eight different size indications for the top leftblock. The lengths shown in the table below were made by evaluating avery large set of images and choosing the combinations (from a verylarge set of possible combinations) that resulted in the lowest overallstorage costs.

Size indication E0 A4 E4 Total Length 000 0 0 0  0 001 2 1 1  4 010 2 33  8 011 4 3 5 12 100 4 5 3 12 101 6 5 5 16 110 6 7 7 20 111 8 8 8 24

The entropy encoding module 2006 chooses the coding from the possibleoptions (e.g. as shown in the table above) with the least total lengththat is able to (losslessly) represent the {E0,A4,E4} triple by removingleading zeroes from the data values. In the event of a tie (e.g. ifdeciding between size indications 011 and 100 in the example shown inthe table above) either tied code could be chosen, but as an example,the code with the least numerical encoding may be chosen (e.g. 011).

The encoded data values for the top left set are included in thevariable size field 2116 (e.g. at the start of the variable size field2116) of the encoded data output 2110 (block 2210).

The remaining (e.g. fifteen) sets of data values in the block 2100 arethen processed (block 2212). Specifically, a number of bits (e.g. aminimum number of bits) for representing the maximum value of the datavalues of a set is determined (block 2214), and an indication of thedetermined number of bits is included in the size indication field 2114of the encoded data output 2110 for the set (block 2216). Block 2214 maybe implemented by finding which of the data values in the set has thesmallest number of leading zeros, and then by identifying the smallestavailable encoding that will suffice. It is noted that not all possibledata lengths may be available to be encoded. For example, as describedin the example below, a data length of seven bits is not available to beencoded. Therefore, the determined number of bits may be a minimum“valid” number of bits which can be used to represent the maximum valueof the received data values in the set, wherein a number of bits isvalid if it can be indicated by an indication to be included in the sizeindication field 2114. For example, there may be a predetermined set ofvalid numbers of bits which can be indicated by said indication (e.g. aslisted in the table above), and the minimum valid number of bits may bethe minimum of the valid numbers of bits in the predetermined set whichis sufficient to represent the maximum value of the received data valuesin the set.

When the number of bits which is to be used to represent each of theencoded data values in a set has been determined, some leading zeroescan be removed from each of the data values in the set (if appropriate)to thereby determine the encoded data values, such that each of theencoded data values in a set has the determined number of bits for thatset.

The encoded data values representing the data values in the set areincluded in the variable size field 2116 of the encoded data output 2110(block 2218). The order in which the encoded data values are included inthe variable size field 2116 is predetermined and corresponds with theorder in which the indications are included in the size indication field2114, such that when the encoded data values are decoded, the positionsof the decoded data values within the block are known.

For example, if the four data values in a set are 00000011, 00000100,00000001 and 00000000, then 00000100 is the maximum value in the set,and three is the minimum number of bits that can be used to representthe maximum value. Three is a valid number of bits in the example shownin the table above in the sense that an indication can be included toindicate that each of the encoded data values of the set have threebits. Therefore, five leading zeroes can be removed from each of thedata values in the set. Therefore, the encoded data values for the setare 011, 100, 001 and 000, and these encoded data values are included inthe variable size field 2116. An indication (e.g. 011) to indicate thatthree bits are used to represent each encoded data value in the set isincluded in the size indication field 2114. Different sets (e.g. 2×2sets) can use different numbers of bits for their encoded data values,and each set has its own indication in the size indication field 1304 toindicate how many bits are used in the variable size field 2116 for theencoded data values of that set.

As an example, the table below shows how the indications may correspondto numbers of bits for the 2×2 sets.

Size indication Number of bits per encoded data value 000 0 001 1 010 2011 3 100 4 101 5 110 6 111 8

It is noted that there is no indication to indicate that seven bits areused for each encoded data value. Therefore, in this example, if themaximum 8-bit value within a set has just one leading zero then theminimum valid number of bits which can be used to represent each of theencoded data values in that set is eight (not seven). This omission ofseven from the predetermined set of valid lengths for encoded datavalues was chosen by examining all the possible choices of number of 3bits to a set of 8 choices. Since there are 9 options (bit lengths 0 to8), one option is left out. Having evaluated all the possibilities (e.g.trialing leaving out 0, or 1, or 2 . . . ) against a large test suite ofimages, it was confirmed that leaving out “7” gives the best level ofcompression. It is noted that the likelihood of data values having onlyone leading zero is small because the colour decorrelation and thespatial decorrelation processes cause the data values to be biasedtowards zero.

It should be apparent that the examples described herein relate to 8-bitdata values, but in other examples, the data values may includedifferent numbers of bits (e.g. 6-bit data values), and a person skilledin the art would be able to modify the disclosure provided herein tooperate with these different numbers of bits.

For example, if each of the data values received from a spatialdecorrelation module has n bits, and the determined minimum valid numberof bits for a particular set of received data values is m, where m≤n,then each of the received data values of the particular set has at least(n−m) leading zeroes. In this case, each of the encoded data values forthe particular set consists of the m least significant bits of acorresponding n-bit received data value of the particular set. Referringto the same example as above, if the four n-bit data values (where n=8)in a set are 00000011, 00000100, 00000001 and 00000000, then thedetermined minimum valid number of bits for representing the maximumvalue of the set, m=3. A data length of 3-bits per encoded data value isa valid encoding from the table above. Each of the data values has atleast five leading zeroes. Therefore, the m-bit encoded data values forthe set are 011, 100, 001 and 000.

Different sets within the block 2100 can be processed in parallel by anentropy encoding module 2006. For example, the indications for thedifferent sets within the block 2100 can be determined and included inthe size indication field 2114 in parallel operations within the entropyencoding module 2006. However, since the encoded data values can havevariable lengths, they are included in the variable size field 2116 in apredetermined sequence.

When all of the encoded data values of the block have been included inthe encoded data output 2110 then, the encoded data output is outputtedfrom the entropy encoding module 2006 (block 2220). The encoded dataoutput representing a block of pixel values is provided to the packingmodule 2008 for each of the colour channels, where they are packedtogether. For example, the packing module 2008 places the encoded dataoutputs for the block of data values from the different colour channelsinto a data packet. In other words, a plurality of encoded data outputsare formed for a respective plurality of colour channels relating to thesame pixels, and the plurality of encoded data outputs for a group ofpixels (e.g. an 8×8 block) are packed together into a data packet forstorage. The encoded data block can then be sent for storage in thememory 106, e.g. via the memory interface 114 (as shown in FIG. 1).

A header is stored with a group of encoded data blocks, e.g. the datablocks representing an image or a frame. For each encoded data block(e.g. each 8×8 block of encoded data values), a header indication isincluded in the header to indicate the size of the compressed data forthe respective encoded data block. The header is stored separately fromthe encoded data values (e.g. in a dedicated portion of the encoded dataoutput), and due to its small size, a significant portion of the headerdata may remain resident in a cache within the GPU. The header allowsthe size of the encoded data block to be known before the encoded datablock is retrieved from memory, so an encoded data block can beretrieved without retrieving extraneous data. This is very useful forallowing random access to different parts of encoded data. In general,any number of bits may be used in the header as a header indication fora respective encoded data block, but in a specific example describedbelow, four bits are used for each of the header indications.

As noted above, whilst the method described above with reference toFIGS. 20A-B, 21A-B and 22 and the method described in UK patent number2530312 are both inherently lossless, if either is used in combinationwith a lossy pre-processing step (e.g. as described above with referenceto FIG. 14 or with reference to FIGS. 16 and 18A when one or more bitsfrom the fourth pseudo-channel are discarded), the resultant compresseddata is no longer lossless.

Any reference to particular logic gates above (e.g. OR, AND gates, etc.)is by way of example only and it will be appreciated that they may bereplaced by logic elements that provide the same functionality and maybe more broadly referred to as logic blocks.

The description above describes many different methods, includingmethods of data compression (e.g. FIGS. 2A-C, 3A, 4-11 and 19A), methodsof bit mapping (e.g. FIGS. 12A-12B), including conversion from 10-bit to8-bit format (e.g. FIGS. 14 and 16). These methods may be usedindividually or two or more of these methods may be used together invarious combinations and several examples are described above. Alsodescribed above are the reverse operations (e.g. data compression,inverse mapping and conversion from 8-bit to 10-bit format) and thehardware for implementing any of these methods or any combination ofthese methods.

FIG. 23 shows a computer system in which the data compression anddecompression methods and apparatus described herein may be implemented.The computer system comprises a CPU 2302, a GPU 2304, a memory 2306 andother devices 2314, such as a display 2316, speakers 2318 and a camera2320. A data compression and/or decompression block 2321 (which mayimplement any of the methods described herein) is implemented on the GPU2304. In other examples, the data compression and/or decompression block2321 may be implemented on the CPU 2302. The components of the computersystem can communicate with each other via a communications bus 2322.

The data compression hardware of FIGS. 2A-C, 12B and 13B are shown ascomprising a number of functional blocks. This is schematic only and isnot intended to define a strict division between different logicelements of such entities. Each functional block may be provided in anysuitable manner. It is to be understood that intermediate valuesdescribed herein as being formed by data compression hardware need notbe physically generated by the data compression hardware at any pointand may merely represent logical values which conveniently describe theprocessing performed by the data compression hardware between its inputand output.

The data compression and decompression hardware described herein(including any hardware that is arranged to implement any of the methodsdescribed above) may be embodied in hardware on an integrated circuit.The data compression and decompression hardware described herein may beconfigured to perform any of the methods described herein. Generally,any of the functions, methods, techniques or components described abovecan be implemented in software, firmware, hardware (e.g., fixed logiccircuitry), or any combination thereof. The terms “module,”“functionality,” “component”, “element”, “unit”, “block” and “logic” maybe used herein to generally represent software, firmware, hardware, orany combination thereof. In the case of a software implementation, themodule, functionality, component, element, unit, block or logicrepresents program code that performs the specified tasks when executedon a processor. The algorithms and methods described herein could beperformed by one or more processors executing code that causes theprocessor(s) to perform the algorithms/methods. Examples of acomputer-readable storage medium include a random-access memory (RAM),read-only memory (ROM), an optical disc, flash memory, hard disk memory,and other memory devices that may use magnetic, optical, and othertechniques to store instructions or other data and that can be accessedby a machine.

The terms computer program code and computer readable instructions asused herein refer to any kind of executable code for processors,including code expressed in a machine language, an interpreted languageor a scripting language. Executable code includes binary code, machinecode, bytecode, code defining an integrated circuit (such as a hardwaredescription language or netlist), and code expressed in a programminglanguage code such as C, Java or OpenCL. Executable code may be, forexample, any kind of software, firmware, script, module or librarywhich, when suitably executed, processed, interpreted, compiled,executed at a virtual machine or other software environment, cause aprocessor of the computer system at which the executable code issupported to perform the tasks specified by the code.

A processor, computer, or computer system may be any kind of device,machine or dedicated circuit, or collection or portion thereof, withprocessing capability such that it can execute instructions. A processormay be any kind of general purpose or dedicated processor, such as aCPU, GPU, System-on-chip, state machine, media processor, anapplication-specific integrated circuit (ASIC), a programmable logicarray, a field-programmable gate array (FPGA), physics processing units(PPUs), radio processing units (RPUs), digital signal processors (DSPs),general purpose processors (e.g. a general purpose GPU),microprocessors, any processing unit which is designed to acceleratetasks outside of a CPU, etc. A computer or computer system may compriseone or more processors. Those skilled in the art will realize that suchprocessing capabilities are incorporated into many different devices andtherefore the term ‘computer’ includes set top boxes, media players,digital radios, PCs, servers, mobile telephones, personal digitalassistants and many other devices.

It is also intended to encompass software which defines a configurationof hardware as described herein, such as HDL (hardware descriptionlanguage) software, as is used for designing integrated circuits, or forconfiguring programmable chips, to carry out desired functions. That is,there may be provided a computer readable storage medium having encodedthereon computer readable program code in the form of an integratedcircuit definition dataset that when processed (i.e. run) in anintegrated circuit manufacturing system configures the system tomanufacture data compression and/or decompression hardware configured toperform any of the methods described herein, or to manufacture datacompression and/or decompression hardware comprising any apparatusdescribed herein. An integrated circuit definition dataset may be, forexample, an integrated circuit description.

Therefore, there may be provided a method of manufacturing, at anintegrated circuit manufacturing system, data compression and/ordecompression hardware as described herein. Furthermore, there may beprovided an integrated circuit definition dataset that, when processedin an integrated circuit manufacturing system, causes the method ofmanufacturing data compression and/or decompression hardware to beperformed.

An integrated circuit definition dataset may be in the form of computercode, for example as a netlist, code for configuring a programmablechip, as a hardware description language defining an integrated circuitat any level, including as register transfer level (RTL) code, ashigh-level circuit representations such as Verilog or VHDL, and aslow-level circuit representations such as OASIS® and GDSII. Higher levelrepresentations which logically define an integrated circuit (such asRTL) may be processed at a computer system configured for generating amanufacturing definition of an integrated circuit in the context of asoftware environment comprising definitions of circuit elements andrules for combining those elements in order to generate themanufacturing definition of an integrated circuit so defined by therepresentation. As is typically the case with software executing at acomputer system so as to define a machine, one or more intermediate usersteps (e.g. providing commands, variables etc.) may be required in orderfor a computer system configured for generating a manufacturingdefinition of an integrated circuit to execute code defining anintegrated circuit so as to generate the manufacturing definition ofthat integrated circuit.

An example of processing an integrated circuit definition dataset at anintegrated circuit manufacturing system so as to configure the system tomanufacture data compression and/or decompression hardware will now bedescribed with respect to FIG. 24.

FIG. 24 shows an example of an integrated circuit (IC) manufacturingsystem 2402 which is configured to manufacture data compression and/ordecompression hardware as described in any of the examples herein. Inparticular, the IC manufacturing system 2402 comprises a layoutprocessing system 2404 and an integrated circuit generation system 2406.The IC manufacturing system 2402 is configured to receive an ICdefinition dataset (e.g. defining data compression and/or decompressionhardware as described in any of the examples herein), process the ICdefinition dataset, and generate an IC according to the IC definitiondataset (e.g. which embodies data compression and/or decompressionhardware as described in any of the examples herein). The processing ofthe IC definition dataset configures the IC manufacturing system 2402 tomanufacture an integrated circuit embodying data compression and/ordecompression hardware as described in any of the examples herein.

The layout processing system 2404 is configured to receive and processthe IC definition dataset to determine a circuit layout. Methods ofdetermining a circuit layout from an IC definition dataset are known inthe art, and for example may involve synthesising RTL code to determinea gate level representation of a circuit to be generated, e.g. in termsof logical components (e.g. NAND, NOR, AND, OR, MUX and FLIP-FLOPcomponents). A circuit layout can be determined from the gate levelrepresentation of the circuit by determining positional information forthe logical components. This may be done automatically or with userinvolvement in order to optimise the circuit layout. When the layoutprocessing system 2404 has determined the circuit layout it may output acircuit layout definition to the IC generation system 2406. A circuitlayout definition may be, for example, a circuit layout description.

The IC generation system 2406 generates an IC according to the circuitlayout definition, as is known in the art. For example, the ICgeneration system 2406 may implement a semiconductor device fabricationprocess to generate the IC, which may involve a multiple-step sequenceof photo lithographic and chemical processing steps during whichelectronic circuits are gradually created on a wafer made ofsemiconducting material. The circuit layout definition may be in theform of a mask which can be used in a lithographic process forgenerating an IC according to the circuit definition. Alternatively, thecircuit layout definition provided to the IC generation system 2406 maybe in the form of computer-readable code which the IC generation system2406 can use to form a suitable mask for use in generating an IC.

The different processes performed by the IC manufacturing system 2402may be implemented all in one location, e.g. by one party.Alternatively, the IC manufacturing system 2402 may be a distributedsystem such that some of the processes may be performed at differentlocations, and may be performed by different parties. For example, someof the stages of: (i) synthesising RTL code representing the ICdefinition dataset to form a gate level representation of a circuit tobe generated, (ii) generating a circuit layout based on the gate levelrepresentation, (iii) forming a mask in accordance with the circuitlayout, and (iv) fabricating an integrated circuit using the mask, maybe performed in different locations and/or by different parties.

In other examples, processing of the integrated circuit definitiondataset at an integrated circuit manufacturing system may configure thesystem to manufacture data compression and/or decompression hardwarewithout the IC definition dataset being processed so as to determine acircuit layout. For instance, an integrated circuit definition datasetmay define the configuration of a reconfigurable processor, such as anFPGA, and the processing of that dataset may configure an ICmanufacturing system to generate a reconfigurable processor having thatdefined configuration (e.g. by loading configuration data to the FPGA).

In some embodiments, an integrated circuit manufacturing definitiondataset, when processed in an integrated circuit manufacturing system,may cause an integrated circuit manufacturing system to generate adevice as described herein. For example, the configuration of anintegrated circuit manufacturing system in the manner described abovewith respect to FIG. 24 by an integrated circuit manufacturingdefinition dataset may cause a device as described herein to bemanufactured.

In some examples, an integrated circuit definition dataset could includesoftware which runs on hardware defined at the dataset or in combinationwith hardware defined at the dataset. In the example shown in FIG. 24,the IC generation system may further be configured by an integratedcircuit definition dataset to, on manufacturing an integrated circuit,load firmware onto that integrated circuit in accordance with programcode defined at the integrated circuit definition dataset or otherwiseprovide program code with the integrated circuit for use with theintegrated circuit.

Those skilled in the art will realize that storage devices utilized tostore program instructions can be distributed across a network. Forexample, a remote computer may store an example of the process describedas software. A local or terminal computer may access the remote computerand download a part or all of the software to run the program.Alternatively, the local computer may download pieces of the software asneeded, or execute some software instructions at the local terminal andsome at the remote computer (or computer network). Those skilled in theart will also realize that by utilizing conventional techniques known tothose skilled in the art that all, or a portion of the softwareinstructions may be carried out by a dedicated circuit, such as a DSP,programmable logic array, or the like.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein. In view of the foregoing description itwill be evident to a person skilled in the art that variousmodifications may be made within the scope of the invention.

What is claimed is:
 1. A lossy method of compressing data comprising:for each sub-block of a block of pixels: analysing an alpha channelvalue for each pixel in the sub-block to select a lossy compression modefrom a set of candidate lossy compression modes, wherein the set ofcandidate lossy compression modes comprises at least one compressionmode which uses a fixed alpha channel value for all pixels in thesub-block.
 2. The method according to claim 1, wherein the methodfurther comprises: for each sub-block of the block of pixels:compressing the sub-block using the selected lossy compression mode,wherein analysing an alpha channel value for each pixel in the sub-blockto select a lossy compression mode from a set of candidate lossycompression modes comprises: determining a minimum alpha channel valueand a maximum alpha channel value from the alpha channel values for allthe pixels in the sub-block; in response to determining that adifference between the maximum alpha channel value and the minimum alphachannel value is larger than a difference value, selecting a compressionmode which uses a variable alpha channel value for the pixels in thesub-block; and in response to determining that the difference betweenthe maximum alpha channel value and the minimum alpha channel value isnot larger than the difference value, selecting a compression mode whichuses a fixed alpha channel value for all pixels in the sub-block anddetermining the fixed alpha channel value.
 3. The method according toclaim 2, wherein the difference value is a maximum error value.
 4. Themethod according to claim 2, wherein determining the fixed alpha channelvalue comprises: in response to determining that the maximum alphachannel value is a maximum possible value, setting the fixed alphachannel value equal to the maximum alpha channel value; in response todetermining that the minimum alpha channel value is zero, setting thefixed alpha channel value equal to zero; and in response to determiningboth that the maximum alpha channel value is not a maximum possiblevalue and that the minimum alpha channel value is not zero, setting thefixed alpha channel value equal to an average of the alpha channelvalues for all pixels in the sub-block.
 5. The method according to claim2, wherein compressing the sub-block using the selected lossycompression mode comprises, for a compression mode which uses a fixedalpha channel value for all pixels in the sub-block: for each of thepixels in the sub-block, converting each of a red channel value, a greenchannel value and a blue channel value, from 8-bits to 5-bits; andpacking the 5-bit red, green and blue channel values for the pixels inthe sub-block and the fixed alpha channel value together to formcompressed data for the sub-block.
 6. The method according to claim 2,wherein compressing the sub-block using the selected lossy compressionmode comprises, for a compression mode which uses a variable alphachannel value for the pixels in the sub-block: sub-dividing thesub-block into a plurality of mini-blocks; for each of the mini-blocks:calculating a colour difference value for each pixel pair in themini-block; comparing a smallest of the colour difference values to athreshold; and in response to determining that the smallest of thecolour difference values exceeds the threshold, converting pixel datafor each pixel in the mini-block from a first format to a second formatcomprising fewer bits than the first format; and in response todetermining that the smallest of the colour difference values does notexceed the threshold, selecting one of a plurality of encoding modesbased on the colour difference values, determining two or three pixelvalues for the selected encoding mode, allocating one of the pixelvalues to each pixel in the mini-block and converting the pixel valuesfrom the first format to a third format comprising fewer bits than thefirst format.
 7. The method according to claim 2, wherein compressingthe sub-block using the selected lossy compression mode comprises, for acompression mode which uses a fixed alpha channel value for all pixelsin the sub-block: sub-dividing the sub-block into a plurality ofmini-blocks; for each of the mini-blocks: calculating a colourdifference value for each pixel pair in the mini-block; comparing asmallest of the colour difference values to a threshold; and in responseto determining that the smallest of the colour difference values exceedsthe threshold, converting pixel data for each pixel in the mini-blockfrom a first format to a second format comprising fewer bits than thefirst format; and in response to determining that the smallest of thecolour difference values does not exceed the threshold, selecting one ofa plurality of encoding modes based on the colour difference values,determining two or three pixel values for the selected encoding mode,allocating one of the pixel values to each pixel in the mini-block andconverting the pixel values from the first format to a third formatcomprising fewer bits than the first format.
 8. The method according toclaim 6, wherein selecting one of a plurality of encoding modes based onthe colour difference values, determining two or three pixel values forthe selected encoding mode, allocating one of the pixel values to eachpixel in the mini-block and converting the pixel values from the firstformat to a third format comprising fewer bits than the first formatcomprises: identifying a pixel pair in the mini-block having a smallestcolour difference (908), representing the pixel pair as a single newpixel value and a pixel pattern code that identifies the pixel pair andconverting pixel data for the single new pixel value and for each otherpixel in the mini-block aside from the identified pixel pair, from thefirst format to a third format comprising fewer bits than the firstformat.
 9. A data compression unit configured to perform the method asset forth in claim
 1. 10. A method of data compression comprising: foreach sub-block of a plurality of sub-blocks, generating compressed datafrom a compressed data block: identifying a compression mode from ablock mode field within the compressed data block, wherein thecompression mode is one of a set of candidate lossy compression modes,the set of candidate lossy compression modes comprising at least onecompression mode which uses a fixed alpha channel value for all pixelsin the sub-block.
 11. The method of data compression according to claim10, further comprising: decompressing the sub-block using adecompression mode corresponding to the selected compression mode.
 12. Adata compression unit configured to perform the method as set forth inclaim
 10. 13. A non-transitory computer readable storage medium havingstored thereon computer readable code configured to cause the method asset forth in claim 1 to be performed when the code is run on at leastone processor.
 14. A non-transitory computer readable storage mediumhaving stored thereon computer readable code configured to cause themethod as set forth in claim 10 to be performed when the code is run onat least one processor.
 15. A non-transitory computer readable storagemedium having stored thereon a computer readable dataset description ofan integrated circuit that, when processed in an integrated circuitmanufacturing system, causes the integrated circuit manufacturing systemto manufacture a data compression unit configured to perform the methodas set forth in claim
 10. 16. An integrated circuit manufacturing systemconfigured to manufacture a data compression unit as claimed in claim10.
 17. An integrated circuit manufacturing system comprising: anon-transitory computer readable storage medium having stored thereon acomputer readable dataset description of an integrated circuit thatdescribes a data compression unit, a layout processing system configuredto process the integrated circuit description so as to generate acircuit layout description of an integrated circuit embodying the datacompression unit; and an integrated circuit generation system configuredto manufacture the data compression unit according to the circuit layoutdescription, wherein the data compression unit comprises: hardware logicarranged, for each sub-block of a block of pixels, to analyse an alphachannel value for each pixel in the sub-block to select a lossycompression mode from a set of candidate lossy compression modes,wherein the set of candidate lossy compression modes comprises at leastone compression mode which uses a fixed alpha channel value for allpixels in the sub-block.
 18. The integrated circuit manufacturing systemas set forth in claim 17, wherein the data compression unit furthercomprises: a data compression element arranged to compress the sub-blockusing the selected lossy compression mode.